As an experienced sendmail professional, you will doubtless find yourself running sendmail by hand fairly often. When you do, many error messages will print to your screen, and will be obvious in context.
But, when run as a daemon, the sendmail program’s error messages will not necessarily be printed to your screen. Most will be logged with the syslog(8) facility (see Section 14.3). When logged, each will be preceded with common logging information, as for example:
Feb 9 08:31:03 yourhost sendmail: f19FQwie023873: putbody: write error ↑error starts here
Here, the common logging information ends with the fourth colon. The error itself follows that fourth colon and is the “putbody: write error.”
Also note that some errors will neither be printed nor logged. They will instead be returned as part of the SMTP dialog (error), as for example:
550 5.1.2 postmaster@korad_media.korad.co.kr... Host unknown
There is no general principle for discovering the error message in SMTP errors. Here, for example, the error message is “Host unknown.”
Another problem is that SMTP errors are generated by the other (remote) site. Because not all sites run sendmail, many such generated error messages will not appear in this Appendix. After all, this is a book about sendmail.