Canary allows you to include a custom signature with each outgoing message.
1) Open Canary.
2) At the upper-left corner of the screen, tap the More Actions icon.
3) In the upper-left corner of the screen, tap the Settings icon.
4) Scroll to the composing section.
5) Tap Signatures.
6) Enter a plain-text signature. (HTML is not supported at this time)
7) To use different signatures for every account, toggle ON Per Account.