Out of the 31 members I have bouncing, all but 3 are Yahoo. In one instance, I looked at the "Email Delivery History" and saw that the response was:
421 4.7.0 [TS01] Messages from 66.175.222.12 temporarily deferred - 4.16.55.2; see ?
The link, of course, is dead, but a little research on the web says that Yahoo Mail will bounce emails with this message if it gets too much activity from the same IP address. You would think that Yahoo would know about Groups.io by this point and enter it as an exception, so maybe they're not changing it on purpose.
