RFC 5322 - Internet Message Format allows multiple, comma separated email addresses in the Reply-To-Header:
In either case, an optional reply-to field MAY also be included, which contains the field name “Reply-To” and a comma-separated list of one or more addresses.
If I click on “reply” in emclient it only uses the very first email from the Reply-To header as the new To: header.
It would be correct to add all addresses from the Reply-To header to the To header of the new email.
Reply All does not do the trick.
Reply All replys to the first email address of the Reply-To header and all email addresses from the FROM / BCC headers. But if a second or third email in the Reply-To header is not part of FROM BCC it is not included at all.
I think it is a bug, not a feature
emClient should respect RFC 5322