Default account signature

You can define a default signature for each of your mail accounts. This can be edited under Edit ▸ Preferences ▸ Mail Accounts ▸ Edit ▸ Identity ▸ Optional Information.

If you have a signature for your account defined but exceptionally do not want to use it for one message, or want to use a different signature, you can change it via the dropdown menu in the right upper corner in the email composer window.