Thunderbird won't accept correct password

Thunderbird demands my password for outgoing SMT server when I go to send an email but when I enter it, system won't accept it even though it is the correct password. I have had this problem for some time and it comes and goes. Sometimes I have ebeen able to send without a problem, then it comes back I had a helpful reply from this forum (can't find who posted it - Mick?) tellling me to go into the error console (Ctrl + Shift + J) and delete eveything in that. I tried that. It worked at first, but now when I do it, it has no effect. And the problem now appears to have become permanent, although occasionally after I have been working for some hours, then suddenly I can send without any problem. Problem hasn't budged though for a couple of days now. This is a mysterious and erratic gremlin, which now seems to have taken root. Any suggestions please? Carol