[CMS of Doom™]
Imagine creating a CMS so bad that you let the owner (who I work for) define in a simple input field what email address is used as the sender address for the welcome email of newly registered users.
Basically they filled in a personal email of the company some 3 years ago AND of course the person with said email address left the company a few months later thus for some 2-3 years newly registered users received a welcome email with a sender address of an unavailable user.

And I thought I've seen it all in this CMS...

So, I don‘t want to defend that CMS because I don‘t even know it but…. How exactly is this the fault of that CMS?

@rEaL-jAsE Man, I used to think that as weall... until I saw this legacy hell hole...

@Lensflare Always assume a user is dumb. You never let them define something critical or important and the sender address of an automated system should always be well defined, either some no-reply address or a generic info@xyz.com, in the first case you'd define an existing reply-to address.
If you let the user define the sender address, chances are, they will either mess up the address or do what happened in my rant. An automated system should never have a personal sender address as it could be misleading for the recipient.
Additionally, there would be the potential of having your automated emails recognized as spam, if the user defines an address from a different domain (SPF problem).

@PonySlaystation I see. Still it seems like a config fail. Unless it‘s a core feature of that CMS.