Debugging sendmail

The -b switch instructs sendmail to "Become"/operate in a mode.
The -d0 switch, instructs sendmail to produce debugging information.
The level .11 prints more information than the default level of .1 .
The -bp switch instructs sendmail to print the contents of the queue.

$ sendmail -d0.11 -bp

You can verify aliases with the -bv switch:

$ sendmail -bv root

To run sendmail verbosely, use the -v switch:

$ cat /dev/null | sendmail -v root

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sendmail host name lookup failure

Check that sendmail resolves corresponding dns with:

echo "/mx smtp.domain.tld." | sendmail -bt -d8.8
echo "/map host smtp.domain.tld." | sendmail -bt -d8.8

If using SMART_HOST which does not resolve or has no MX record, then make sure to include the host/IP with square brackets and sendmail will not try to do a dns lookup, else you would get "host name lookup failure" in maillog.

In "":

define(`SMART_HOST', `[smtp.domain.tld]')dnl

Apply the config:

m4 /etc/mail/ > /etc/mail/

Restart Sendmail:

service sendmail restart