When mta.backend is set to use sendmail (either with 'mail' or 'sendmail' values), sendmail automatically inserts the required Date header. However, when it's set to SMTP, the Date header is never inserted.
[http://www.faqs.org/rfcs/rfc2822.html] section 3.6: "The only required header fields are the origination date field and the originator address field(s)."
When mta.backend is set to use sendmail (either with 'mail' or 'sendmail' values), sendmail automatically inserts the required Date header. However, when it's set to SMTP, the Date header is never inserted.
[http://www.faqs.org/rfcs/rfc2822.html] section 3.6: "The only required header fields are the origination date field and the originator address field(s)."