Service Desk Email FROM address mangled with equal sign

[Dynatron Software]

We are using our mailgun account to send SMTP email from service desk.  All mail going out has a terrible FROM/REPLY address of

support=mydomain.com@mailgun.mydomain.com

Instead of support@mydomain.com

So if anyone replies to an email sent from service desk it of course bounces.

I use this mailgun account with my own scripts and apps constantly and simply force a FROM and REPLY-TO address of support@mydomain.com and it all works great. 

What is causing this issue?

[Andy Heinzer]

I suspect the problem here is either with the settings Jira is using, or with the way the SMTP server requires usernames to be formatted.

Could to go into the system settings of Jira (Gear Icon -> System -> Outgoing Mail)?  I am interested to see what values you have set for both From and Username here.   I know that other email provides, like gmail, started requiring logins to use the full email address, such as abcd@example.com instead of just abcd.  This is important for mail providers that are managing multiple possible domains through the same system.  However with gmail at least, it isn't including the entire username before the domain again.   So it might be something wonky this particular SMTP provider is doing in order rewrite some of the headers of the message when it is being sent out.

Also, does this also happen for non-service desk projects, like a Jira Core Business project or a Jira Software project?   I ask because it's possible Jira Service Desk might be using slightly different code to handle outbound notifications that Jira sends out.  I'm just curious to see if the scope of this problem extends itself beyond just Service Desk or not.