A few clients of ours have been experiencing this problem lately. Looks like some of the email services are dropping some emails if they dont like the look of them. Reading around it seems that if an email hits a number of criteria the email carrier will not simply place it in junk / spam but drops the whole email with no warning or feed back to user.



The fix that got us moving again was to activate the return path for all emails sent.

System > config > advanced > system > mail sending settings > set return path > YES

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