When a member joins I get a notice and I approve them
Then I get a undelieverable report as a server error
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I presume you're trying to approve them by email.
The following recipient(s) cannot be reached:
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????? 'Groups.io' on 11/27/2018 1:36 PM
??????????? Server error: '553 5.1.3 Syntax error in recipient address'
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The problem is that your reply is being sent to "" - which is not a valid address. There should have been (a lot) more to the return address. It should have looked something like:
Possibly the hyphens in your group name, or the very long resulting email address confused your email interface and the Reply function didn't copy the address correctly.
Next time you try to approve one by email, before you click send make note of the address your reply is being sent TO. If it doesn't look like the above report back here what's being put there. Also tell us what email interface you're using.