What does this all mean?

E-mail incoming sums the total size and number of messages for mailbox holders on your post office that has been received by your domain.

E-mail outgoing sums the total size and number of messages sent via your post office (using niptech.net's SMTP service) by your mailbox holders.

E-mail internal are the total size and number of messages sent via your post office by your mailbox holders (using niptech.net's SMTP service) to other mailbox holders in your domain.

Virus counts are the number of E-mail coming into your domain from external E-mailers and the number of attempted sends of infected E-mails from your mailbox holders. You can't do much about the incoming infected E-mail, but outgoing infected E-mail indicates that one of your mailbox holders has an infected PC.

Addresses are the total number of mailboxes on your domain. As can be seen on the detailed report we can separate addresses into separate groups for billing and analysis.

Mbytes (MB) is approximately one million bytes. This is the amount of E-mail (data), which was received by your post office and forwarded to the intended recipient's mailbox for collection.