A BM server has a public facing network card with an address of
xxx.xxx.xxx.2, and several secondary addresses of xxx.xxx.xxx.3 and
xxx.xxx.xxx.4 and xxx.xxx.xxx.5 etc.

Will it always send outgoing packets, in this case to a broadband router
on xxx.xxx.xxx.1 from its primary xxx.xxx.xxx.2 address ?

It may not seem relevant but outgoing mail from the BM server to a mail
relay server needs to be filtered based on its source address from the BM