OT - what does nmsgs tell me?

[Steve Campbell]

I've googled, but can't find exactly what the nmsgs part of a log entry 
in my maillog is telling me. For example, see below:

Mar 22 13:21:10 mailserver ipop3d[17789]: Login user=mailuser 
host=fwx128.cnpap
ers.net [216.12.119.190] nmsgs=11/21

I don't understand what the "11/21" means. I thought it meant 11 of 21 
messages were deleted, which meant 10 remained on the server, but that 
doesn't make sense to me.

Any answer would be appreciated.

Steve Campbell