by josh » Fri Feb 02, 2018 4:59 pm
It may be that the sender in this case has a DMARC policy in place that instructs your local email provider to discard the incoming messages because they're not coming from the sender's mail server (they're coming from the spamgourmet mail server instead, of course). I'm just guessing - but this is happening a lot recently.
If you do not have "Reply Address Masking" enabled, try turning it on, and that may solve the problem for new incoming messages. As flippant as this sounds, if you have Reply Address Masking turned on, you can try turning it off, and that may help. The possible combinations of DMARC directives and the way your mail server interprets them can result in problems (and solutions) either way.