I have a webservers that send out emails to a sendmail relay server as a batch job. The emails need to be accepted by the relay sendmail server as fast as possible, however, they do not need to go out (be relayed) very quickly.

I am seeing a couple timeouts once and a while from the webserver trying to connect to the relay server. The load currently is about 30 emails a second for a couple minutes.

There are quite a few tuning options for sendmail in the sendmail tuning guide.

What I am focusing on now is the Delivery Mode:

Delivery Mode

There are a number of delivery modes that sendmail can operate in, set by the DeliveryMode ( d) configuration option. These modes specify how quickly mail will be delivered. Legal modes are:

i deliver interactively (synchronously) b deliver in background (asynchronously) q queue only (don't deliver) d defer delivery attempts (don't deliver) There are tradeoffs. Mode i gives the sender the quickest feedback, but may slow down some mailers and is hardly ever necessary. Mode b delivers promptly but can cause large numbers of processes if you have a mailer that takes a long time to deliver a message. Mode q minimizes the load on your machine, but means that delivery may be delayed for up to the queue interval. Mode d is identical to mode q except that it also prevents lookups in maps including the -D flag from working during the initial queue phase; it is intended for ``dial on demand'' sites where DNS lookups might cost real money. Some simple error messages (e.g., host unknown during the SMTP protocol) will be delayed using this mode. Mode b is the usual default. If you run in mode q (queue only), d (defer), or b (deliver in background) sendmail will not expand aliases and follow .forward files upon initial receipt of the mail. This speeds up the response to RCPT commands. Mode i should not be used by the SMTP server.

I currently have the CentOS default modes:

Sendmail.cf:

DeliveryMode=background

Submit.cf:

DeliveryMode=i

• Is sendmail.cf/mc for outgoing email from relay (to the intertubes) and sumbit.cf/mc for incoming eamil (from my webservers).
• Would it make sense to change the outgoing delivery mode to queue? If I did, what would the outbound email flow behave like?
• If this is the right thing to do, can anyone show me example mc configurations for this change? If it isn't, what recommendations are there for these constraints?