One of our users clicked "unsubscribe" on a Maniphest task and got the following error message:

Notice: Undefined index: HTTP_REFERER in /opt/sendgrid/site/releases/80dcee6f57f4346abe4744a3aa2fda2f8719f320/web/standalone.php on line 389
Unsubscribe successful

Your email XXXX@signifyd.com has been successfully unsubscribed from this list!

But he continued to receive email alerts for the task. So he went back and clicked the button again and got the same error message. Now he's not getting any email alerts for anything. I checked and his email preferences have email turned on for everything. Even went into the DB and copied my preferences over his, since I am getting emails:

set @v1 := (select user_preferences.preferences from user_preferences, user where userPHID=phid and userName = 'oliver');
update user_preferences set preferences=@v1 where userPHID = 'HIS-PHID';

I then verified both in the web gui and in the DB that my settings had been applied. However, he's still not getting emails. We double-checked his spam folder and I ran ./bin/mail list-outbound and ./bin/mail show-outbound and mail seems to think that it is delivering the messages, but they simply don't arrive any more.