Determines start position of message in file.

A Long value.  

object.StartPosition [= value]

The StartPosition Property syntax has these parts:

If you keep more than one message in a file, you should let Pop3 server know what are starting and ending positions of particular mail message in the mailbox. If such information is provided, on user's request for that particular message, Pop3 server will just send data between those values and will ignore the rest.

This way you can have, for instance, one mailbox for each user (or even one mailbox for all users) and just provide info about starting and ending positions and Pop3 server will parse it properly. However, keep noted that storing all mail messages in one mailbox will make deleting (in UpdateMailbox event) much more difficult than just deleting files that store one message.