If you reach out to support, don't be too surprised if they can't help you too much. I've had mixed results with support. Sometimes they are amazing, sometimes they come back and tell you they can't help you.
Worst scenario I had was that we couldn't get the server to recognize that we reconfigured it to have more JVM memory. We updated the configs, rebooted the server, and it was stuck at default of 4 GB (I think... may have been 2 GB). We reached out to support, sent them our configs, and their response was "you must have something configured on the physical server. We cannot support that. Talk to your server admin to make sure they don't have anything limiting the JVM max memory". I get it; they don't have access to our servers, but we obviously talked to the server admin prior to this and during and they indicated that wasn't the case. Later, another admin on the system (who is both a front end and server side admin) was reviewing the config file and saw a typo. They corrected the typo and we had more max memory! Atlassian support was not very helpful and was quick to "pass the buck".
In your scenario, it honestly wouldn't surprise me if support asks you to switch to PostgreSQL!
On a similar note - I should say that my setup is with Jira Server. As yours is a new install (from end of April), and you have access to the database, you must be running Data Center as you cannot buy Server licenses anymore. I am not sure if Data Center edition does funky things with the mail, so it MAY not be a misconfiguration thing... it MAY be something stupid that data center is trying to do. I'd still recommend opening a support case - you are paying for support, so might as well use it. But do be prepared to wait. I find it usually takes 24 hours for first reply and 24-72 hours between replies.
The above is all just my opinion on what you should do.
As with all advice you find on a random internet forum - you shouldn't blindly follow it. Always test on a test server to see if there is negative side effects before making changes to live!
I recommend you NEVER run "random code" you found online on any system you care about UNLESS you understand and can verify the code OR you don't care if the code trashes your system.