Philippe Lehoux and for shared accounts the account owner would have to set those aliases, right? You can't set an alias for a shared account if the account is merely shared with you it seems, but I might miss something?

In Missive, you can setup any number of aliases for each of your email accounts, and each alias can have a different signature. See attachment image to know where to add aliases the in Accounts settings.

Even if you don’t have different alias addresses, you can create multiple aliases with the same Address and Name, just for the purpose of setting a different signature for each.