Even if you don't encrypt every mail you send, signing is a good idea. Is it unnecessary in many circumstances? Yes, but at least I find it nice to be able to verify authenticity. I don't understand why my bank (Wells Fargo) can't figure out how to sign all their emails [1].
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1: https://www.wellsfargo.com/downloads/pdf/com/cps/Secure_Emai...
Yes, they can sign some emails...but it requires someone inside WF "sponsor" you to be added to their PKI and it still won't lead to signed or even encrypted emails for online banking. WF is also particularly egregious in not offering real 2FA...they count a username AND a password as "two factor authentication". sigh