How do you set these up in Thunderbird? "Alias" is a somewhat slippery word; maybe these would work as additional identities on the one account.

The description given here leads me to think that an alias should be treated as an additional identity. On the account settings page, under "Account Settings" look for the the "Manage Identities" button. Setting them up here will allow you to send using these as "From:" addresses, but will respect the fact that all incoming messages addressed to an alias will land in your primary account's mailbox.

Problem has been solved but not by using the solution above but thank you for trying. The problem was largely due to finding my way around Tbird and thinking that my other postboxes had the same password.

Thank you for sharing. Your experience may help others in a similar situation.