[paid] help to fix outgoing email issue

[ambimorph]

"There are 3865 email jobs that failed. Check your app.yml and ensure that the mail server settings are correct."

At some point a few weeks ago email started failing. I haven’t had time to address it until now, and I’m at a bit of a loss. There was a similar problem here: Error: "There are 965 email jobs that failed. Check your app.yml and ensure that the mail server settings are correct." that was solved by rebooting the server. I tried rebooting the droplet, but that doesn’t seem to have helped. What else can I try?