Is a Reply-To header field required, and is "Reply-To: <>" valid?
1. This newsgroup is about the sendmail program, not generic e-mail
2. There are RFCs that specify the format of e-mail, e.g., RFC 2822 et.al.
Have you checked those?
[Note the follow-up to group.]
In addition to that which is stated in the RFC, certain other "valid" constructs will be treated as spam.
E.g. "Reply-To: <email@example.com>" - Use of the ".invalid" TLD in an optional header is a clear sign of spam. That is not in any RFC (other than implied
in RFCs 2606 and 6761).
A "null" mailbox such as "<>" would also be considered spam. A null mailbox
is valid only in the SMTP MAIL FROM statement, not message headers.