I have a gitlab-ce AWS instance (t3.large) that according to documentation is more than enough to run Gitlab but for some reason I keep receiving email notifications days after they should have been sent.
I’m using the default Gitlab email settings (GitLab CE version 12.5.4) and I myself am using gmail.
I also tried to switch to an instance with higher specs.
I honestly don’t recall these settings. And posting them helps others to see your configuration and start analysing from here, without anyone requiring to do extra work.
Granted, that you’re using these settings, you likely have access to the mail server sending these emails. Analyse its logs and figure out why these emails are delayed, bounced, etc. Highly likely the mail server doesn’t have an MX record, or is mistrusted in a different way.