1. Using a different gmail account from the account used to set up a test group called Curt-Test, I sent an note to Curt-Test+subscribe@groups.io 

2. Got this reply
"Confirm Your Groups.io Subscription. 
Thank you for your interest in Groups.io and the group Curt-Test@groups.io. To complete your subscription, please reply to this email.

3. Ignored the instructions and clicked on the link, which sent a note to the group.  

4. Got this response from Gmail:
"Delivery Status Notification (Failure)

Message not delivered

Your message couldn't be delivered to Curt-Test@groups.io because the remote server is misconfigured. See technical details below for more information.

The response from the remote server was:

510 5.1.1 Your email address, <redacted>@gmail.com, is not subscribed to that group. To subscribe, send an email to Curt-Test+subscribe@groups.io, or visit https://groups.io/g/Curt-Test "

5. When I ignored the link and sent a reply to the Confirmation email, it worked fine.
The address in the reply was tagged:
"Groups.io" <Curt-Test+subscribe+939633873206536861+5163825@groups.io>

Issue:
When I looked closely at the confirmation email, I saw that the link was 

mailto:Curt-Test@groups.io
-- in other words, causing the applicant to submit a post to a group that they are not yet a member of.

Suggested fix:

The mailto link provided in a confirmation email should have been 
Curt-Test+subscribe+939633873206536861+5163825@groups.io
-- i.e. matching the reply address built into the confirmation email.

Thanks!
--cg