A bug was filed for opensmtpd-portable regarding escape sequences inside
quotes in headers: <https://github.com/OpenSMTPD/OpenSMTPD/issues/1242>.

The issue is easily replicable by sending a mail with a from as follows:

	From: "\"Doe, John\"" <op>

smtpd mis-interprets the escapes inside the quoted name and turns it
into:

	From: "\"Doe@localhost, John\"" <op>

When smtpd reaches the comma it thinks it's the address separator and
not part of the string.  This is caused since the \ is treated as
literal in a quoted context and so \" closes the quoted string from
smtpd point of view.

If I'm reading RFC5322 correctly, it actually allows for escapes inside
quotes.  Citing bits of the ABFN:

   name-addr       =   [display-name] angle-addr
   display-name    =   phrase
   phrase          =   1*word / obs-phrase
   word            =   atom / quoted-string
   quoted-string   =   [CFWS] DQUOTE *([FWS] qcontent) [FWS] DQUOTE [CFWS]
   qcontent        =   qtext / quoted-pair

diff belows allows for handling of escapes in quoted strings.  It only
affects how To, Cc and From headers are processed.

Thoughts/oks?

diff /home/op/tmp/smtpd
commit - d84d1565c6d01997768b2220610a26af306be2d9
path + /home/op/tmp/smtpd
blob - 94763e6d07801384a6f00a612e61fa8f56998cad
file + smtp_session.c
--- smtp_session.c
+++ smtp_session.c
@@ -489,7 +489,7 @@ header_domain_append_callback(struct smtp_tx *tx, cons
 				quote = !quote;
 			if (line[i] == ')' && !escape && !quote && comment)
 				comment--;
-			if (line[i] == '\\' && !escape && !comment && !quote)
+			if (line[i] == '\\' && !escape && !comment)
 				escape = 1;
 			else
 				escape = 0;