In July 2021 messages I sent to @gmail.com started being returned. It took a little troubleshooting but I realized my signature was causing the problem. Apparently, Google has started blocking all emails that use bit.ly short links. I removed “Twitter: https://bit.ly/TweetMikeBiddle LinkedIn: https://bit.ly/MikeBiddleLinkedIn” from my signature and immediately messages were delivered without any problems.
I’m not sure if this has been reported yet, but I hope this helps someone out there with the same issue! I first noticed Google was blocking bit.ly links on 7/10/2021.
Delivery has been delayed to these recipients:
Subject: Test Message 3
Remote Server returned: ‘421 4.7.0 [00.00.000.00 15] Our system has detected that this message is 4.7.0 suspicious due to the nature of the content and/or the links within. 4.7.0 To best protect our users from spam, the message has been blocked. 4.7.0 Please visit 4.7.0 https://support.google.com/mail/answer/188131 for more information. bl28si2552811qkb.351 – gsmtp’