We support up to 1GB for all mail boxes.
There are 3 cases where your mail box could become full:
Using IMAP - keeping all of your mail on the server to be accessible to all of your devices and mail clients can lead to the box filling up. We suggest using POP
Setting your mail client (such as Outlook or Mac Mail) to leave all mail on the server even when using POP will lead to your mail box getting full. We suggest setting a limit to leave mail on the server for up to 2 week after it has been downloaded to your mail client.
Using webmail only - If you only use webmail your mail box is not being downloaded anywhere and will need to be periodically emptied so that you do not reach your limit.