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.

You can turn this off in the settings and the spam will go in a Junk folder.

https://kb.mailbox.org/en/private/e-mail-article/customizing...

I'm referring to outbound email being silently dropped, not inbound email being rejected or put into Junk.

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.

Unfortunately this is common in many smtp servers and is configuration dependent: After you authenticate as usera@example.com you can send emails as userb@example.com.