4.3 I am a dial-up user and I've done everything right so far (I think). I can get my mail just fine with fetchmail and mail works locally as it should. The trouble is that my user name on my local account is not the same as my user name at my ISP so my mail goes no where when I send it. What can I do to fix this?

Aside from changing your local user name to match your ISP account name there is a simple fix for this which works quite well. Here's what to do:

A canonical mapping should be all that is needed but you have to edit main.cf so that Postfix actually looks at the table by adding this line to /etc/postfix/main.cf:

   canonical_maps = hash:/etc/postfix/canonical
   

Add something like the following to /etc/postfix/canonical:

   username@hostname user@ISP
   

Be sure to run postmap /etc/postfix/canonical after each change. Then run postfix reload.

What the canonical map does is rewrite the address, in any message it processes, when it sees a match. In our example from above if the map saw the address on the left it would rewrite it to the one on the right.