Fix false FORGED mail header 'FROM' field detection/rejection

Pretty much any From field that contains an '@' in the username portion
was subject to comparison to the full email address, but clearly some of
these rejected emails were not forge attempts:

'Chris @ StubHub' vs 'events@mail.stubhub.com'
'Eric S. Raymond (@esr)' vs 'gitlab@mg.gitlab.com'

Fixed by requiring that the sender name is actually a well-formed Internet
email address using smb_netaddr_type(), which was also recently improved to
be more accurate.

Unrelated change: include reverse-path (email address for bounces) in
ILLEGALLY-LONG body and header line log messages (usually SPAM from what
I can tell).