Look at the /etc/services file. It lists the port numbers for all the various “standard” protocols.
SMTP is outgoing email (pushing to a mailbox from a client).
POP3 is incoming email (i.e. fetching email from a mailbox to a client).
IMAP is also incoming email (i.e. fetching email from a mailbox to a client). Most setups use only one of IMAP or POP3.
You need to redirect these ports only to do what you're asking.
But there may be other ways. Can you set up 192.168.0.5 to use the Linux router as a gateway? Then it could just connect directly to 192.168.7.2 to fetch or send email, and the gateway should take care of it by the non-default route (the default route is only ever used as a fallback).
You could also look at port forwarding and tunneling programs. Personally, I use stunnel on my computer to tunnel and secure the POP3 socket on my local machine to the POP3S socket on my mail server. The mail client then fetches my email from the local machine — at which point, neither the email client nor NAT cares about the fact that the email actually comes from a different computer.
Hope that helps,
— Robert J. Lee