2 questions following your post:
a) what about the destination MAC address?
If I’ve understood what you said, I guess it is:
destination: POP3 IP
MAC source: MACA
MAC destination: MACB
source: a fake IP on the same net, i.e. 192.168.0.100
destination: must be your IP on that adapter / network, in our example 192.168.0.10
MAC source: MACB
MAC destination: MACA
b) do I send the modified packets to Adapter or MSTCP?