Comment by dsissitka

Another thing is that they appear to have some spam scanning on outbound emails and when they detect something suspicious they simply drop the email silently, and nobody will ever know about it.

Oh, thank you. I recently considered moving from posteo.de to mailbox.org, but I think I won't anymore regarding such an issue took so long to even be considered as a problem and as I understand is still not solved.

Oh, thank you. I didn't know that. Anyway, I'm not using a custom domain on mailbox, I use my custom mail domain with another service.