Error when trying to send mail to user group

i have a user group called “alle-i-klyngen” and i have prior been able to send mail from the mail app, just by typing this user group name… but not all of a sudden my mail is stuck in the outbox and i get error “could not send massage”

ive tried sending the same mail test to a single mail and also sent same mail to 4 reciepients and that works fine …

what to do?

my nextcloud is hosted at IONOS.de so i dont have access to terminal nor am i root user…

how can i troubleshoot on this issue?