I am able to select my certificate to digitally sign email but on the same screen i cannot select the same certificate under the 'Encryption' heading. The 'Choose A Certificate' dropdown is greyed out. Additionally On Certificate Authentication option - i see only two com.apple.idms.... certificates but not mine. 

I've copied my certificate into Microsoft_Entity_Certificates and Microsoft_Intermediate_Certificates in keychain - but this has had no effect. 

I can however work fine in Apple Mail - sending digitally signed and encrypted emails without any problem.