This is not "a group rule" but I'm posting about this because your own email address is something you don't want random bots that scour the net to be able to grab for phishing and other scamming purposes.
The NVDA Group archives are public, and, thus, not only indexed by search engines like Google, DuckDuckGo, StartPage, etc., but can also be crawled by any other individual or entity that wishes to do so. One of the most common ways to harvest email addresses is by crawling the web and looking for those posted out in the open, in all their glory. Even some of the older tricks like separating the first part of the address, at sign, and last part of the address cannot be ensured as being effective as the address is very simply reconstructed from same.
Every member can do as they see fit. I have generally "cleaned" messages of email addresses if I happen to read them shortly after they've been sent in hopes that the archive has not yet been crawled and the address harvested.
My recommendation on those occasions where you wish to be contacted off-group is to refer to the Reply to Sender link at the bottom of each message from Groups.io. Activating that should fire up your email mechanism of choice, be it an email client like Outlook or webmail, with the subject pre-populated with the prefix, Private:, followed by the topic title for the message itself and that message sender's email address in the To.
Your email address is not exposed using this method, as those links at the end of Groups.io messages that are automatically generated do not get saved in the archive. They're a part of individual email messages only.
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
Always remember others may hate you but those who hate you don't win unless you hate them. And then you destroy yourself.
~ Richard M. Nixon