Comparison of Outbound Email Delivery Strategies

1
Comparison of Outbound E-mail Delivery Strategies

• Postini SMTP Proxy, Smart Hosts, Direct Delivery
• January 14, 2008

2
Direct Delivery of E-Mail Messages
Transfer Agent
Retreival Agent
Transfer Agent
Message Sender
User Interface
Mail Queue
User Mailbox
User Interface
Submission Agent
Transfer Agent
Access Agent
Message Recipient
Mail Client
Originating Server
Destination Server
Mail Client

• User composes message and submits message to
  originating server
• A limited amount of error checking is done in the
  client and server
• The client can check for basic proper formation
  of recipient addresses
• The originating server might check for validity
  of remote domains in DNS
• The client or originating server can validate
  local addresses against directory
• Once the message leaves the client, status is via
  mail notification
• Servers will send status notification messages
  back to the sender as needed

3
Direct Delivery of E-Mail Messages
Transfer Agent
Retreival Agent
Transfer Agent
Message Sender
User Interface
Mail Queue
User Mailbox
User Interface
Submission Agent
Transfer Agent
Access Agent
Message Recipient
Mail Client
Originating Server
Destination Server
Mail Client

• The originating server places the message in a
  queue for holding
• Queued messages are delivered when the
  destinations can be reached
• If a destination is available, the message will
  typically be delivered instantly
• If not reachable, the message will remain queued
  on the originating server
• In Exchange, a separate queue is maintained for
  each destination server
• To improve efficiency, the server keeps track of
  state of destinations
• If a destination is known to be down, retries are
  attempted less frequently

4
Direct Delivery of E-Mail Messages
Transfer Agent
Retreival Agent
Transfer Agent
Message Sender
User Interface
Mail Queue
User Mailbox
User Interface
Submission Agent
Transfer Agent
Access Agent
Message Recipient
Mail Client
Originating Server
Destination Server
Mail Client

• The destination server accepts the message and
  delivers to mailbox
• Delivery failures can still occur at this point
  for various reasons
• Mailbox size quotas or individual message size
  limitations may exist
• Filtering mechanisms may reject a message based
  on its content
• Failures should be detected before the message is
  completely accepted
• Destination servers should not generate delivery
  status messages
• The sender address on a message may be faked,
  generating backscatter

5
Direct Delivery Characteristics

• Advantages to the originating mail server
  administrator
• High degree of visibility into the email delivery
  process
• No additional components or outside services
  needed
• Simple, minimal, configuration minimizes points
  of failure
• Disadvantages to the originating mail server
  administrator
• Server and IP address are directly visible on
  Internet
• Server administrator needs to handle resolution
  of blacklist issues
• Administrator needs to perform monitoring and
  troubleshooting
• Mail delivery may be affected by connectivity of
  mail server
• Server needs to be able to handle size of queues
  and retries

6
What is a Smart Host?

• A Smart Host is simply a fully-functional SMTP
  email server
• Smart Hosts accept messages and deliver them to
  their final destinations
• A Smart Host takes all responsibility for a
  message once it accepts it
• Smart Hosts handle queuing, delivery retries, and
  delivery failures
• The Smart Host is an old concept dating to the
  start of SMTP email
• Not all machines had enough power to route email
  messages
• Dumb Hosts simply forwarded all messages to a
  Smart Host
• Smart Hosts may perform additional message
  processing
• Smart Hosts might filter outgoing messages to
  prevent egress of spam
• Outgoing messages might be tagged as an aid in
  filtered inbound mail
• Smart Hosts have become increasingly necessary
  due to spam
• Consumer and small business ISPs often will not
  allow direct email delivery
• Forcing outbound mail through Smart Host allows
  monitoring and control

7
Smart Host Mail Delivery Process
Transfer Agent
Retreival Agent
Transfer Agent
Transfer Agent
Message Sender
User Interface
Mail Queue
Mail Queue
User Mailbox
User Interface
Submission Agent
Transfer Agent
Transfer Agent
Access Agent
Message Recipient
Mail Client
Originating Server
Destination Server
Mail Client
Smart Host

• User composes message and submits message to
  originating server
• A limited amount of error checking is done in the
  client and server
• The client can check for basic proper formation
  of recipient addresses
• The originating server might check for validity
  of remote domains in DNS
• The client or originating server can validate
  local addresses against directory
• Once the message leaves the client, status is via
  mail notification
• Servers will send status notification messages
  back to the sender as needed

8
Smart Host Mail Delivery Process
Transfer Agent
Retreival Agent
Transfer Agent
Transfer Agent
Message Sender
User Interface
Mail Queue
Mail Queue
User Mailbox
User Interface
Submission Agent
Transfer Agent
Transfer Agent
Access Agent
Message Recipient
Mail Client
Originating Server
Destination Server
Mail Client
Smart Host

• The originating server places the message in a
  queue for holding
• Queued messages are sent to the Smart Host almost
  immediately
• No routing decisions are made except whether a
  recipient is local or remote
• As long as Smart Host is available, the
  originating queue remains empty
• In Exchange, one queue is maintained as the Smart
  Host is one destination
• The Smart Host may perform additional validation
  at acceptance time
• If a message is rejected, the originating server
  sends a status to the sender

9
Smart Host Mail Delivery Process
Transfer Agent
Retreival Agent
Transfer Agent
Transfer Agent
Message Sender
User Interface
Mail Queue
Mail Queue
User Mailbox
User Interface
Submission Agent
Transfer Agent
Transfer Agent
Access Agent
Message Recipient
Mail Client
Originating Server
Destination Server
Mail Client
Smart Host

• The Smart Host places the message in a queue for
  holding
• Queued messages are delivered when the
  destinations can be reached
• If a destination is available, the message will
  typically be delivered instantly
• If not reachable, the message will remain queued
  on the Smart Host
• The Smart Host is responsible for queue
  management and delivery retries
• The originating server no longer has any
  responsibility for the message
• If a message is delayed or fails, the Smart Host
  sends a status to the sender

10
Smart Host Mail Delivery Process
Transfer Agent
Retreival Agent
Transfer Agent
Transfer Agent
Message Sender
User Interface
Mail Queue
Mail Queue
User Mailbox
User Interface
Submission Agent
Transfer Agent
Transfer Agent
Access Agent
Message Recipient
Mail Client
Originating Server
Destination Server
Mail Client
Smart Host

• The destination server accepts the message and
  delivers to mailbox
• Delivery failures can still occur at this point
  for various reasons
• Mailbox size quotas or individual message size
  limitations may exist
• Filtering mechanisms may reject a message based
  on its content
• Failures should be detected before the message is
  completely accepted
• Destination servers should not generate delivery
  status messages
• The sender address on a message may be faked,
  generating backscatter

11
Smart Host Characteristics

• Advantages to the originating mail server
  administrator
• The originating server does not need to perform
  queue management
• Monitoring is simplified, server becomes almost
  maintenance free
• Originating server is isolated from the Internet
  remains anonymous
• Delivery troubleshooting is handled by the Smart
  Host administrator
• Resolution of blacklisting is handled by the
  Smart Host administrator
• Outgoing bandwidth reduced for multiple-recipient
  messages
• Disadvantages to the originating mail server
  administrator
• Loss of visibility into the delivery process
  cant status messages
• Smart Host is a potential additional point of
  failure in delivery
• Traffic from other Smart Host users could
  increase blacklist risk

12
What is the Postini SMTP Proxy?

• Postini provides outbound mail handling as part
  of their service
• The service includes filtering of outgoing
  messages for viruses
• Provides anonymity of IP address as mail going
  out under Postinis address
• The service is implemented as an SMTP proxy, not
  a Smart Host
• Postini does not queue messages, data is simply
  passed through in real time
• Any failure conditions from destination are
  passed back to originating server
• For delayed deliveries, the originating server
  needs to handle retries
• Exchange does not support configuration of an
  SMTP proxy
• The Postini services needs to be configured as a
  Smart Host on Exchange
• Because it is not truly a Smart Host, this causes
  some negative effects

13
Postini Mail Delivery Process
Transfer Agent
Retreival Agent
Transfer Agent
Transfer Agent
Message Sender
User Interface
Mail Queue
User Mailbox
User Interface
Transfer Agent
Submission Agent
Transfer Agent
Access Agent
Message Recipient
Mail Client
Originating Server
Destination Server
Mail Client
Postini Proxy

• User composes message and submits message to
  originating server
• A limited amount of error checking is done in the
  client and server
• The client can check for basic proper formation
  of recipient addresses
• The originating server might check for validity
  of remote domains in DNS
• The client or originating server can validate
  local addresses against directory
• Once the message leaves the client, status is via
  mail notification
• Servers will send status notification messages
  back to the sender as needed

14
Postini Mail Delivery Process
Transfer Agent
Retreival Agent
Transfer Agent
Message Sender
Transfer Agent
User Interface
Mail Queue
User Mailbox
User Interface
Transfer Agent
Submission Agent
Transfer Agent
Access Agent
Message Recipient
Mail Client
Originating Server
Destination Server
Mail Client
Postini Proxy

• The originating server places the message in a
  queue for holding
• Queued messages are delivered to destination
  through Postini
• No routing decisions are made except whether a
  recipient is local or remote
• If a delivery fails or is deferred, the message
  stays queued at the origin
• In Exchange, one queue is maintained as Postini
  is setup as a Smart Host
• Errors from the destination are treated by
  Exchange as errors from Postini
• The message passes through Postini but is not
  stored by them

15
Postini Mail Delivery Process
Transfer Agent
Retreival Agent
Transfer Agent
Message Sender
Transfer Agent
User Interface
Mail Queue
User Mailbox
User Interface
Transfer Agent
Submission Agent
Transfer Agent
Access Agent
Message Recipient
Mail Client
Originating Server
Destination Server
Mail Client
Postini Proxy

• The destination server accepts the message and
  delivers to mailbox
• Delivery failures can still occur at this point
  for various reasons
• Mailbox size quotas or individual message size
  limitations may exist
• Filtering mechanisms may reject a message based
  on its content
• Failures should be detected before the message is
  completely accepted
• Destination servers should not generate delivery
  status messages
• The sender address on a message may be faked,
  generating backscatter

16
Postini SMTP Relay Characteristics

• Advantages to the originating mail server
  administrator
• Originating server is isolated from the Internet
  remains anonymous
• Resolution of blacklisting is handled by Postini
  service
• Outgoing mail is scanned for viruses
• Disadvantages to the originating mail server
  administrator
• Administrator needs to perform monitoring and
  troubleshooting
• Interjection of Postini in mail path complicates
  troubleshooting
• Exchange cannot manage the queue optimally with
  Postini
• Server needs to be able to handle size of queues
  and retries
• Postini service is a potential additional point
  of failure in delivery
• Traffic from other Postini users could increase
  blacklist risk

17
Comparison Between Methods

• Exchange SMTP queues
• Postini One queue for all messages, problem
  deliveries mixed in
• Direct One queue per destination, only problems
  will have a queue
• Smart Host One queue for all messages, queue
  will remain empty
• Content filtering
• Postini Service provides content scanning of
  outgoing mail
• Direct Any scanning would be performed on
  originating server
• Smart Host Service might or might not provide
  scanning of mail
• Visibility of servers
• Postini Mail appears to come from Postini
  addresses
• Direct Mail would show IP address of originating
  server
• Smart Host Mail appears to come from the Smart
  Host

18
Comparison Between Methods

• Delivery performance
• Postini Problem destinations might affect
  delivery of other messages due to shared link
  state and intermingling of messages in queues
• Direct Problem destinations will only affect
  those destinations since each message is
  maintained in a separate queue
• Smart Host Messages leave server immediately and
  queue stays empty, Smart Host manages delivery of
  messages and retries
• Monitoring and troubleshooting
• Postini Server administrator performs delivery
  troubleshooting, having Postini as an
  intermediary complicates diagnosis
• Direct Easy to monitor for issues, server
  administrator needs to perform all
  troubleshooting of deliveries
• Smarthost Very little that administrator can do,
  Smart Host performs all monitoring and
  troubleshooting