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JMail is a class alias to Joomla\CMS\Mail\Mail set during the application boot. Typehinting a method parameter works equally well no matter if you type hint against the FQN (fully qualified namespace) class or its alias. For a proof that this works on all versions of PHP supported by ATS please refer to this quick test on 3v4l.

Here's the thing. While Joomla registers a class alias it doesn't meant that a third party plugin can't register its own concrete class under this name! The problem is that now JMail is a concrete class which does not extend Joomla\CMS\Mail\Mail, therefore the type check fails. And here's the 3v4l proof of concept.

This is NOT a bug in Akeeba Ticket System. It's a bug in a third party plugin which is incompatible with Joomla 3.8 and later. If you really want to go down the completely misguided approach of completely overriding the system mailer (VERY BAD IDEA! COMPATIBILITY, FUNCTIONAL AND SECURITY BUGS AHEAD!) you need to override JMail in Joomla 3.7 or lower and Joomla\CMS\Mail\Mail on Joomla 3.8 and later since JMail was replaced by and became an alias to Joomla\CMS\Mail\Mail. By not doing that you have TWO system mailers, JMail and Joomla\CMS\Mail\Mail, which are incompatible with each other. Depending on whether the developer is using Factory::getMailer or Mail::getInstance() they'd end up with a different object. Therefore the plugin that caused this situation is incompatible with Joomla 3.8 or later. Oh, trust me, I made the mistake of buying one of them and it took me all of 18' before I deleted it and started writing my own.

Since this is a public ticket, let's break it down to the two most likely culprits and discuss the alternatives.

Plugins which convert plain text email to HTML. These are the most typical and worst offenders since they date back to the late Joomla 2.5 / early 3.1 days with minimal changes ever since. They override JMail causing the mailer class multiple personality issue described above. They also fail to incorporate security changes made to Joomla's mailer around 3.7.1 with regards to validating email addresses. If you want to see how it should really be done you can take a look at my MailMagic plugin. I use it on my personal blog site among other places. It needs a bit more polishing and some extra features before I can release it for free :)

Support for third party mailer services. Seriously, people, you do not need a third party plugin to have your site send emails with Amazon SES or any other third party email service. All of them support SMTP. Joomla supports SMTP too. Don't shoot your feet to avoid having to tie your shoelaces.

I hope that gives you more insight into what is going on. I can tell you that I'm not changing how ATS works because what I do is correct. What the third party extension does is wrong. That's what needs to be fixed instead of me breaking ATS :)

You're welcome!

To be fair, before you filed this ticket I didn't think it was possible to declare a concrete class with the same name as an already defined class alias. I had to test it with 3v4l before I could be sure that the crazy thing I thought was going on was actually happening.

Please do file a bug report with the third party developer of the problem extension. I believe that they probably don't know about this either. What PHP is doing is rather esoteric. From their perspective the extension didn't break in J3.8 and they didn't have to change it... but it is actually broken in a subtle and very important way.