M4 Language

To be fair, 75% of the problems I saw people have with sendmail.cf over a decade or two was because they didn't know much if anything about M4 and tried to take some template they found and beat it into the shape they needed. Those who I made at least read the O'Reilly M4 book (and the sendmail book) had better success.

The other 25% of the problems I saw people have with sendmail.cf was because it's a horrorshow. Yes, if you need to process an RFC822 email over UUCP and wrap it up and send it over an SNA link to an IBM PROFS email server (for example), sendmail is probably your tool. But who has done that since the 90s?