disroot SMTP server rejecting email


Complete newbie here. I have just signed up and have got email working with the disroot IMAP server, but seem to have some problems with the SMTP server.

If I send an email from my disroot email name, the SMTP server accepts it,
But if the sender address is one of my other (non disroot) email addresses, then it is rejected with a message like:

SMTP error from remote mail server after RCPT TO:someone@somewhere.com: 553 5.7.1 another@xxxxxxx.com: Sender address rejected: not owned by user witwall3

I am used to SMTP servers accepting any reasonable sender, given proper login and security to guard against spammers.

Is this a limitation of the disroot SMTP service, or is this (one of) the point of having disroot email aliases?

Disroot does not allow you sending as anyone. You can only send emails form addresses you own (so your email address or aliases you own). This is to prevent situation when you could easily spoof others.