PHP 8 - Mailhog and CI 4.1.3

I've upgraded my CI4 to the latest version (4.1.3), no real problems there, and now want to use PHP 8 for future development work. So far nothing unreasonable.

However, my mailhog installation works fine with PHP 7.4.21, but the exact same code does NOT work with PHP 8.0.8.

This is perplexing. I don't know where the error is, it seems like the intercept part that mailhog performs is not happening. The mail appears to send from the mail command correctly, but nothing shows up anywhere. It's like it vanishes into the magic bitbucket.

Any thoughts would be appreciated.