S\Mime certificates emClient for MacOS

Can someone advise please eM Client fro MacOS support S\Mime certificates?  It allows to import certificate but unfortunately I can’t  send an encrypted by these certificates email and I got a message that my S\MIME certificates are not valid.

I’m ready to buy Pro version of eM Client  because I’m found it  very suitable for my tasks  but I need to be sure that I can work with emails encrypted by S\MIME certificates.

Thank You in advance

Ask the recipient to provide you with a PGP public key, or you can import their’s from a public key server if they have published it.