This is a local mailer issue, not a sendmail issue. Depending on what you're doing, look at procmail, ftpmail, Majordomo or of course Vacation, which is part of the sendmail (8.10.0 and later) distribution.

The latest version of Majordomo can be found at https://www.greatcircle.com/majordomo/. It is written in Perl and requires either Perl 4.036, and appears to run with only minor tweaks under 5.001a or later. Make sure to check out the web interface for Majordomo called LWGate. The latest versions of Perl (both 4.x and 5.x) can be found at https://www.perl.org/get.html. More information about Perl can be found at that link as well.

The latest version of ftpmail can be found at https://www.filewatcher.com/m/ftpmail.el.Z.4175-0.html or any comp.sources.misc archive.