Found the reason, but for explaining I have to somewhat go far afield.
tl;dr: sending mails did work the whole time, only not to domains registered in yunohost.
long version:
Since I’m not masochistic enough to host my own mail server, I configured the nameservers of my domain hoster such, that web is routed to the yunohost but mail is directed to my mailhoster. I checked and double checked that sending and receiving mails to the email adress that belongs to my Glitch-soc installation in the email client, and since it did, I had no reason to monitor the yunohost internal mailbox.
That was a mistake, because obviously yunohost rewrote the domainpart of the email adress to the main domain of my yunohost installation.
Instead of sending mail to accountname@my_glitchsoc.example yunohost changed the recipient to accountname@my_yunohost.example and filed the mail away in the internal yunohost mailbox instead of looking up the recipient in my mailhosters nameserver like it does with emails to other recipients.
But at least this only concerns mails from Glitch-soc to my admin account while all other emails will look into the usual DNS and deliver the mail accordingly to my mailhoster.
Edit: can I close the topic myself or does that have to be done by an admin?