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Jim,

Look at the email address it said it could not deliver to:
greecianhoney@...
That is not a valid email address.

True.

Groups.io now disguises the sender's email addresses.

Not entirely correct.

That's not a disguise, that's a work-around for the DMARC problem created by some email services. You'll note that it only applies to members using certain email services.
/static/help#dmarc

It also applies to yourself if you use Gmail, but that's for a different reason - it avoids a "spoofing" warning from Gmail.

Shal might jump in here and tell us but there is a return address and
there is a reply to address and the two may not be the same.

Correct.

In groups where Reply To is set to "Group", Groups.io includes a Reply-To header field specifying that replies should be sent to the group's posting address. This would be different than the From: field.

At any rate, what is causing your problem is that Outlook is using
that disguised address as the reply to address and it is not a valid
email address by design.

Correct again.

Some email interfaces, and apparently Office 365 is one of them, ignore the Reply-To header field and instead send the reply to the From address.

The Reply-To field is a part of the email standard, and has been from the start.
" The originator fields also provide the information required when
replying to a message. When the "Reply-To:" field is present, it
indicates the address(es) to which the author of the message suggests
that replies be sent. In the absence of the "Reply-To:" field,
replies SHOULD by default be sent to the mailbox(es) specified in the
"From:" field unless otherwise specified by the person composing the
reply."
RFC 5322 Section 3.6.2

IMO ignoring Reply-To should be treated as a bug in the email interface.

Shal


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