Not a valid email address because it is not of the form user@host
From MozillaZine Knowledge Base
This error message can occur if you're trying to use email addresses without a domain such as <lastname.firstname> on a corporate internet. Thunderbird expects the full email address.
The normal workaround is to use the Allow Local Addresses extension. That seems to be the best solution if your SMTP server isn't a normal Internet mail server.
You could also use the Config editor and set the preference mail.identity.default.autocompleteToMyDomain to true. You could also replace "default" with an identity specifier (for example, id3) to make settings that are specific for certain identities.