As far as I understand it, the relay servers only hold the messages until they are delivered, but what happens if they can never get delivered? Does the server just endlessly accumulate these messages in limbo, and never purge them, or are they on a timer? Even if they are on a timer, if a group chat with one abandoned user has a large enough volume of messages, they could theoretically overwhelm the server's storage.

Update

I have received an official response on this from the SimpleX Chat team:

the servers hold the messages for 21 days if they are not received