Sendmail Operation Guide

Timeouts

All time intervals are set using a scaled syntax. For example, 10m represents ten minutes, whereas 2h30m represents two and a half hours. The full set of scales is:

s: seconds
m: minutes
h: hours
d: days
w: weeks

Queue interval

The argument to the -q flag specifies how often a sub-daemon will run the queue. This is typically set to between fifteen minutes and one hour. RFC1123 section 5.3.1.1 recommends that this be at least 30 minutes.

Read timeouts

Timeouts all have option names Timeout.suboption. The recognized suboptions, their default values, and the minimum values allowed by RFC1123 section 5.3.2 are:

connect: The time to wait for an SMTP connection to open (the connect(3sock) system call) [0, unspecified]. If zero, uses the kernel default. In no case can this option extend the timeout longer than the kernel provides, but it can shorten it. This is to get around kernels that provide an absurdly long connection timeout (90 minutes in one case).
iconnect: The same as connect, except it applies only to the initial attempt to connect to a host for a given message [0, unspecified]. The concept is that this should be very short (a few seconds); hosts that are well connected and responsive will thus be serviced immediately. Hosts that are slow will not hold up other deliveries in the initial delivery attempt.
initial: The wait for the initial 220 greeting message [5m, 5m].
helo: The wait for a reply from a HELO or EHLO command [5m, unspecified]. This may require a host name lookup, so five minutes is probably a reasonable minimum.
mail[1]: The wait for a reply from a MAIL command [10m, 5m].
rcpt[1]: The wait for a reply from a RCPT command [1h, 5m]. This should be long because it could be pointing at a list that takes a long time to expand.
datainit[1]: The wait for a reply from a DATA command [5m, 2m].
datablock[1]: The wait for reading a data block (that is, the body of the message) [1h, 3m]. This should be long because it also applies to programs piping input to sendmail which have no guarantee of promptness.
datafinal[1]: The wait for a reply from the dot terminating a message [1h, 10m]. If this is shorter than the time actually needed for the receiver to deliver the message, duplicates will be generated. This is discussed in RFC1047.
rset: The wait for a reply from a RSET command [5m, unspecified].
quit: The wait for a reply from a QUIT command [2m, unspecified].
misc: The wait for a reply from miscellaneous (but short) commands such as NOOP (no-operation) and VERB (go into verbose mode) [2m, unspecified].
command[1]: In server SMTP, the time to wait for another command [1h, 5m].
ident: The timeout waiting for a reply to an IDENT query [30s, unspecified]. On some systems the default is zero to turn the protocol off entirely.
fileopen: The timeout for opening .forward and :include: files [60s, none].
hoststatus: How long status information about a host (e.g., host down) will be cached before it is considered stale [30m, unspecified].

Footnote:

[1]: For compatibility with old configuration files, if no suboption is specified, this timeout is set to the indicated value.

Many of the RFC1123 minimum values may well be too short. sendmail was designed to the RFC822 protocols, which did not specify read timeouts; hence, versions of sendmail prior to version 8.1 did not guarantee to reply to messages promptly. In particular, a RCPT command specifying a mailing list will expand and verify the entire list; a large list on a slow system may easily take more than five minutes.

NOTE: This verification includes looking up every address with the name server; this involves network delays, and can in some cases can be considerable.

SCO recommends a one hour timeout: since a communications failure during the RCPT phase is rare, a long timeout is not onerous and may ultimately help reduce network load and duplicated messages.

For example, the lines:

O Timeout.command=25m O Timeout.datablock=3h

sets the server SMTP command timeout to 25 minutes and the input data block timeout to three hours.

Message timeouts

After sitting in the queue for a few days, a message will time out. This is to insure that at least the sender is aware of the inability to send a message. The timeout is typically set to five days. It is sometimes considered convenient to also send a warning message if the message is in the queue longer than a few hours (assuming you normally have good connectivity; if your messages normally took several hours to send you wouldn't want to do this because it wouldn't be an unusual event). These timeouts are set using the Timeout.queuereturn and Timeout.queuewarn options in the configuration file (previously both were set using the T option).

Since these options are global, and since you can not know how long another host outside your domain will be down, a five day timeout is recommended. This allows a recipient to fix the problem even if it occurs at the beginning of a long weekend. RFC1123 section 5.3.1.1 says that this parameter should be ``at least 4-5 days''.

The Timeout.queuewarn value can be piggybacked on the T option by indicating a time after which a warning message should be sent; the two timeouts are separated by a slash. For example, the line:

OT5d/4h

causes email to fail after five days, but a warning message will be sent after four hours. This should be large enough that the message will have been tried several times.