An issue tracker for reporting bugs or for making feature requests is now available for MailMate. The “simple” URL is https://tracker.mailmate-app.com/tickets, but it is going to redirect to a page hosted by Lighthouse.
Version 0.9.2 of MailMate has just been released. You could try it out now and then report any problems by creating tickets in the issue tracker.
If you are looking for a simple bug/issue-tracker for public and/or private usage, you should definitely take a look at Lighthouse. It is even free to use for small projects. And while you are at it, also take a look at their sister product Tender Support. Both of these services can be controlled via email which is of course perfect for a developer of an email client.