You may encounter an issue where Kayako is throwing an error stating that the email message was rejected by the recipient's mail server due to the mailbox is full.
The "mailbox full" error message simply means that the recipient's mailbox is full (may have exceeded the memory size limit) and can no longer hold any messages.
The recipient needs to delete old messages or move them to a different folder to make room for new messages.
NOTE: This cannot be resolved on Kayako's side. Kindly contact the recipient using a different channel to inform them about the error, and look into this issue at their end.