Your Conversations page can hold a lot of messages:
For free members, the size limit it: 5,000
For A-list members, it's unlimited.
If you're seeing an error that you have a full mailbox, you may want to upgrade to A-list to get more space.
🎉 Fun fact: even if you only purchase one month of A-list and cancel right away and go back to free after that month, you'll keep the bigger mailbox size. We're just nice like that.
If you're trying to send a message but are getting the error that they have a full mailbox, unfortunately there isn't any way to get around that. You'll just have to wait until they upgrade to get more space.
If you still need help, you can email a friendly human.