I take it that you don't have this problem when you sign in to Thunderbird to get your mail -- i.e. it's only when you try to '''send''' mail? It's possible that your password is being forgotten or is somehow corrupted. Go to Preferences > Security > Passwords > Saved Passwords and check that the password for the outgoing (SMTP) server is correct. Also check (under Account Settings) that the correct SMTP server is being used for this account. If that's all OK it may be that your connection to the server is being slowed/interrupted from time to time. The fact that the message sends 'later' without any difficulty is significant. If you are on a home or business network, for example, could the times you experience this problem coincide with times of high demand by other users?