Intro
Thinking of replacing sendmail? Or switching to sendmail? Here are 54 features I find useful in the way I implement sendmail.
In poorly ordered listing, we have:
– minimal acceptable delivery speed: 100 messages/sec
– queue deletion after 3 days (customizable)
– customizable timers on:
– time to wait for initial connection
– time to wait for response to MAIL command
– time to wait for response to QUIT
– time to wait for response to RCPT TO command
– time to wait for response to RSET command
– time to wait for response to other SMTP commands
– ability to turn off identd usage
– customizable greeting
– ability to deliver local mail for error situations such as looping mail, invalid sender + invalid recipient
– ability to detect looping messages and log and remove them
– errors in MIME format
– configurable maximum message size
– configurable maximum number of recipients per message
– configurable minimum queue age before delivery is re-tried
– configurable address operator characters
– ability to set multiple names for this host
– support for alias address transformations
– support for domain aliasing
– configurable load average at which new messages are refused
– configurable load average at which new messages are queued for later delivery
– configurable load average at which SMTP responses are delayed
– ability to run TLS as server and client, and use a CA-issued certificate
– use of fast table lookups to efficiently handle tables with thousands of entries
– configurable mail relaying decisions based on recipient domain
– ability to turn off UUCP routing
– ability to avoid canonification of recipient domain
– ability to re-write sender address
– ability to make mail relaying decisions based on sender address as well as sender domain
– ability to allow only selected domains/IPs/subnets to relay mail
– ability to reject messages to specified recipients/domains with custom message
– ability to silently discard messages to specific recipients/domains
– ability to discard or reject messages from specific senders or sender domains
– ability to set custom error number for rejected email
– support for mass-import and mass alteration of table entries (e.g., to mail routing/access/alias lists)
– ability to restrict mail relaying to all but a positive match list of IP addresses, subnets and FQDNs
– ability to accept unresolvable domains
– ability to run multiple instances, each with independent configuration, with separate IPs, on same appliance
– ability to make mail routing delivery decisions based on recipient domain configurable by MX lookup, set IP address, FQDN with and without MX lookups
– ability to route all else via DNS lookup
– ability to include comments within the configuration
– ability to turn off ESMTP delivery agent to selected domains and act as simple SMTP delivery agent
– ability to send hourly reports
– log available in real time
– log containing at least these fields: sender, recipients, date/time, delay, size, messageId, TLS used flag, sending MTA, relay MTA, reject reason (if applicable)
– ability to analyze logs with RegEx
– ability to archive logs for up to three months
– ability to send test message through itself with customizable subject on periodic basis
– ability to report on queue contents by top sender/recipient/recipient domain
– ability to force delivery retry on selected domain
– ability to set greeting delay for selected IPs and subnets
– ability to run a browser from same IP as used by the MTA
Most, but not all, of these features are in configured in the .mc file. A few are actually reference to external programs I developed. A few rely on the Linux environment that sendmail runs under.
Conclusion
When you sit down and document it, there’s a lot going on in sendmail.