It may help to specify the email provider. For example, Gmail provides the password, not the user.

I guess I should have been more clear. The question relates to Thunderbird. Firstly I am asking the question because I have started to get messages from Thunderbird that it is unable to connect to the account but no reason stated. Later I seem to be getting the email updates.

Under "Settings", Privacy & Security" there is a place to see "Saved Passwords". There you could see for each Provider what the Username and saved Passwords were by clicking on "Show Passwords". I have 4 email accounts on Thunderbird. For 3 of them there are 2 entries one looks like my password but also another entry that starts with "M.C107_SN1.-C......". These entries are the most recent "Last Changed". How can I be sure what the actual password is?

Those unusual and, to us, unknown, passwords, are created by the email provider and stored there, not to be changed by us. It is a special encoding the email provider does to uniquely identify that PC. My guess, and that's all it is, is that any password errors you receive may be temporary errors on the network, since you otherwise do receive the messages.