Or 552 5.2.3 Message exceeds fixed maximum message size
If this error message is coming from VPOP3 when you are sending a message, it means that the message you are sending is prohibited by your VPOP3 settings. There are two places where the size limit could be set.
In Services → SMTP Server → General - the Maximum Message Size setting
In the user settings → Permissions tab - the Maximum Outgoing Message Size setting
In either of these places, a setting of 0 (zero) means no limit, however that is generally not recommended, as email is not the best system for sending large files, and there are other systems available for sending large files to other people (eg http://www.sendyourfiles.com). In general we recommend not allowing messages over about 50MB.
Note that the sizes you specify are in bytes as the message is sent. Any attachments will be usually encoded using a system called Base64, which makes attachments about 33% larger, so a 15MB attachment will become 20MB when encoded.