As reported in bug 245813, mail prompts for the master password when checking an account with no password stored. This started between linux trunk 2003092205 and 2003092305, indicating the checkin for bug 219976 is the culprit.
This bug seems like a dup of 247417, no? (This one seems clearer :-)
ugh, yes. oh well. *** This bug has been marked as a duplicate of 247417 ***
Verified as dup.