[tombrossman]

Try switching to an email service that supports custom Sieve scripts, which you can use to permanently reject messages based on variables you configure.

These are handled differently than message user agent filtering. Incoming messages are immediately rejected and the sending server is notified.

It's much easier than trying to contact some company that doesn't bother validating email addresses. You already know they are technically deficient so just bounce everything. Problem's at their end, let them work it out.

Fastmail do this, as do a few other hosted email providers. Highly recommended. I also use Sieve filters to reject attachment types beyond the default set, such as Microsoft Office files (.docx, .doc, etc.).

Here's some documentation to get started. No affiliation, just a happy customer. https://www.fastmail.help/hc/en-us/articles/1500000280481-Si...