sporadic problem sending: stuck in outbox

[Linda]

we just changed from Sympatico to our local cable company
Cogeco, we of course deleted Sympatico mail account and
created Cogeco as the default. We have no others. We
sometimes get our messages stuck in outbox. When we hit
Send/receive we get a box that asks us if we want to use
the default account as there is not a valid account for
sending messages. This is puzzling.Clicking yes sends the
message no problem. Is there a way to bypass this> all
our account info is correct and it doesn't always occur.
thanks