o per rfc821, rfc5321 and others there should be an CRLF after each mail-headers line,
so change '\n' to '\r\n'
o enclose mail addresses between '<>' (rfc821)
o swap From: line to DisplayName <mail address> (rfc5312)
Add various mail headers to suppress auto-replies (out of office ...)
o add rfc2076 header (Precedence: bulk)
o add rfc3834 header (Auto-Submitted: auto-generated)
(this header is already honored by dovecot2)
o add special header for MS Exchange (X-Auto-Response-Suppress: OOF, DR, RN, NRN, AutoReply)
patch can be grabbed additional from here:
How-To-Repeat: - replace sendmail with another mailer and do a strict rfc821 enforce.
- implement a vacation mail on your main mailbox (dovecot2/MS-Exchange/...) and let a couple of machines send cron mails to your mailbox