'separated by newlines' seems more idiomatic than 'one per line'
introduce parens for 'See mmsg(7)...'. This phrase ('See mmsg(7)...') appears in a lot of the man pages, and I just want to pin down its usage style a bit. So I'm proposing using it in brackets. In one or two of the other man pages, I'll also be proposing to move it from where it is currently.
lose comma, and trailing semi-colon, unless you say we need the trailing semi-colon in from\&:sender\&:reply\&-to\&:to\&:cc\&:bcc\&:?
'the' standard input. Standard input appears in a few man pages, and I would like to standardise on using 'the' standard (input|output), as God intended.
'addr-spec address' is as per RFC 2822, so I don't think there's any need for 'addr-spec part of the address' or 'the addr-spec component' or similar
from\&:sender\&:reply\&-to\&:to\&:cc\&:bcc\&:
?