OK a customer domain set up on Server(a) with all mail services along with basic email throughput throttling. The domains MX points to Server(a).
Some customers need to send larger volume of emails (bulk mail) so we configured another Server(b) SMTP only and MX set to External. Added their domain and created the same email accounts that require the facility as on Server(a) i.e. accounts @ domainname.com
So accounts @ domainname.com using Server(b) emails 5000 invoices and say cc's an email address on server(a) it seems that each email sent is then counted as an Outgoing message on Server(a) even though it was sent from Server(b). Actually any emails sent from Server(b) to any domain on the same server then its counted as Outgoing message.
If you use an email account on Server(b) that does not exist on Server(a) the Outgoing message is then counted as part of "_other"
Hope that made sense