>> Should & substitution be done on all parts, or only the full name proper? > > I don't know. I'll ask on Unix Stack Exchange. Well, I didn't get an answer from there but GitHub code search is awesome. A simple search for "gecos" across the entire FreeBSD source tree and behold: sendmail expands that stuff! <https://github.com/freebsd/freebsd/blob/1d6e4247415d264485ee94b59fdbc12e0c566fd0/contrib/sendmail/libsm/mbdb.c#L206> Thie is probably as canonical as anything. Delivering the world's mail traffic provides ample test data and mail is a high priority to fix. So it splits GECOS into fields - not only at commas, but also at semicolons and percent signs. Perhaps as a security measure if those are problematic for mail headers. It expands an arbitrary number of ampersands in the first GECOS field (but the output buffer size is limited; if it overflows, sendmail just uses the Unix username instead of the full name). Since sendmail only needs the first field, we unfortunately don't get an answer to ampersands in the other fields.
