Delete old messages from server [can't send messages when mailbox full]

[ian]

The exact logic has changed in the past few months, but basically now incoming mail on chatmail should purge the oldest existing message from your mailbox, creating space as needed.

I don’t think that sent messages are stored at all, so your quota should not influence sending.

Do you have any error message, log entry or screenshot to share about this?

Related:

Automatically deleting old messages [from the server]

If you use single-device only (or if you have disappearing messages enabled), your messages will already be deleted. I think the better proposal would be to delete messages from the server immediately after all of your devices have fetched them.

Deleting Delta Chat's e-mails on server without deleting messages in Delta Chat

If you use a chatmail account, incoming messages will be purged after about 14 days by your server. If you only use a chatmail account with a single device, Delta Chat is supposed to delete all messages from the server after reception. If you used the backup feature in the past, this toggle might have switched, you can disable multi-device mode under advanced settings.

Latent change in server settings [delete messages from server]

If you are using single-device mode, your messages are already deleted from the server upon fetching. If you are using multi-device mode, the messages can’t be deleted, as then the other clients couldn’t fetch them. Also, chatmail servers have a retention that already deletes messages after a few weeks.