I think perhaps the best thing to do is to add a checkbox, separate from Reply To, that when checked will munge all From lines in messages to a group. Perhaps call it 'Convert All Email From Addresses To Be From Groups.io' with the help text being: "Only use this feature if you use Office 365 and have had complaints from users about email spoofing.'

Submitted my request more than two hours ago...no response. I guess when you have 468 members in beta and all of them try to do this at the same time, it takes a little longer than 10 minutes. :-)

We have a number of users who are either using email via on-premise Exchange, hosted Exchange, or Office 365 business/enterprise.  Exchange checks both the envelope-From address (SMTP FROM) and the header-From address (actual From address in email) for both SPF and internal domain blocking.  While this is not true of all mail server software, it is true of all Exchange server versions which encompasses the vast majority of enterprise and business email servers.  Recently (within the past year), Office 365 finally implemented DMARC on their servers for North America customers but not all customers have had the knowledge or initiative to properly implement it.

With that background, I am appreciative of the group.io team in implementing auto-change on the header From address for detected DMARC reject domains.  However, most Exchange installations do not implement DMARC because it would need to be implemented via third-party software and would require DNS changes. Most Exchange email users do not have access to any of this to make alterations and therefore are stuck with emails they originate getting bounced when sent back through groups.io or having other peoples' emails getting bounced due to SPF failure.  While you may disagree with how Microsoft implemented SPF and domain validation, it is what it is and has been this way for over a decade (don't expect any changes now).

I am requesting that, in addition to the Reply to Group setting, we have the ability to turn on the "via groups.io" setting for all users in a group.  This would allow group owners to resolve this issue with business-class mail servers for their individual group(s) without adversely affecting other groups.