Exchange should already provide the userSMIMEcertificate. It would be very helpful to have this attribute mapped to the LDAP attributes userCertificate and userSMIMEcertificate. That allows Thunderbird to select the right certificates for encryption.
I almost closed this one... However it seems that userMimeCertificate is indeed available over EWS, see:
https://docs.microsoft.com/en-us/exchange/client-developer/web-service-reference/usersmimecertificate
Based on this blog https://docs.microsoft.com/sk-sk/archive/blogs/mstehle/whats-new-for-ews-in-exchange-2010-sp2 userCertificate is mapped to contacts:MSExchangeCertificate.
See pull request https://github.com/mguessan/davmail/pull/98
Last edit: Peter Krútel 2020-04-08
An updated pull request is at https://github.com/mguessan/davmail/pull/353 based on Peter's work above.