doesnt outlook do what you say Thunderbird does ?
i have 4 accounts that i pickup e-mail from all on different servers (infact all with different isp's)
there is a problem though most isp's wont let you send and e-mail that has to go through another isp.
for example if you are with btinternt and collect e-mail from talktalk tiscali and pipex which is not a problem,
bt will not let you route a reply throught them and then from talktalk.
if the op is talking about 5 e-mail addresses all on btinternet then you can collect and reply to each individual email on the account it came in on via outlook...