Digitally sign the message internet explorer outlook owa

I use exchange 2016 and outlook owa, I have s / mime configured, loaded certificates.Signed or encrypted letters from other users are decrypted correctly.When I want to send a letter and digitally sign, the "Digitaly sign this message (S/MIME) or "Encrypt this message (S/MIME) are grayed out (not active).I'm using internet explorer 11.