Recently at our business, starting in about November, we started having ‘delay’s’ of email coming to our Exchange on premise server. It got worse as time went on, and email could be delayed from 6 hours to 24 hours.
We diligently searched google and found many references to check – including our MX records, SRV records, TLS protocol on the Exchange server and even some items for our Barracuda spam filter. And there were volumes of information stating that it was delayed at a Gmail server (using MXtoolbox). None of those solutions solved our problem.
The last piece of equipment that never crossed our minds was the Sonicwall Firewall. It turned out that the Sonicwall has a BLACKLIST company configured by default. And that blacklist company had gone out of business – (with several of the Google gmail servers evidently listed at the time as spam sources).
Once we disabled that blacklist setting – ZOOM all the gmail instantly showed up.
The setting can be found on the Sonicwall here: Anti-spam / RBL Filter. Just make sure the companies listed there are still in business!