Server-side signature
A signature added on a mail server after the message is sent, so it appears whatever device sent it.
With server-side signatures, outgoing mail passes through a rule or a third-party service that adds the signature after the sender presses send. Exchange mail flow rules and tools such as CodeTwo, Exclaimer and Rocketseed work this way.
The advantage is coverage: mail from phones, shared mailboxes and apps is signed too. The trade-offs are that the sender does not see the signature while writing, and mail is routed through the service that adds it.