Sending messages over the Internet is a surprisingly complex undertaking. In addition to the simple SMTP transaction, hostnames need to be looked up with DNS, your address needs to be looked up by the receiving site, and your site can be probed with ident. In this section, we explore ways that you can improve performance over the network.
Avoid spam behavior. If you try to send too many messages to a single site too quickly, that site can artificially slow you down.
Use a well-exercised name server. MX records timeout and need to be looked up periodically. A busy name server will keep them fresh, whereas a special bulk-email sending name server will find that many records have timed out when it is asked to run.
Reverse lookups of your address must be swift. Every time you connect to another site to send email, that site looks up your address before accepting email.
Don’t let the identd(8) port hang. Many sites will use the ident protocol to see who at your site is sending the email. If you want to prevent that lookup, do so by rejecting the connection, instead of letting the connection hang.
You are limited by the smallest piece of the pipe. Installing a gigabit network in-house will not speed outbound email if you are connected to the Internet with DSL.
The sendmail program is written to send email in a fast but well-behaved manner. For example, sendmail will try, whenever possible, to send all messages to a single site sequentially, reusing ...