S/Mime encryption issue

I have an S/MIME issue both in version 7 and 8.

Version 7.2.37929.0
Version 8.0.3385 (83a873c)

My E-Mail for example is aa@aa.com and the receipients adress is bb@bb.com
With aa@aa.com i use emclient. When i send an e-mail to bb@bb.com the e-mail
is encrypted with the s/mime certificate for aa@aa.com an not for bb@bb.com.
So, bb@bb.com received this e-mail with .p7m attachement.
When bb@bb.com (not emclient) send an e-mail to aa@aa.com, the e-mail is correkt
encrypted with the public key for aa@aa.com.

When i send an e-mail from aa.aa.com to bb@bb.com with my Smartphone E-Mail Client
and the same Certificates, bb@bb.com receives this e-mail correctly encrypted.

Any ideas ?

How do you know which public key was used?

Yes, if the message is encrypted, the receiver will get it as a blank message with an attachment, something like encrypted.eml.p7m

When i open the sent e-Mail, there is a line with the information that the email was signed und encrypted. When i click on “encrypted” a pop shows that it is the certificate from the sender (aa@aa.com).
The encryption Mailgateway log, in front of bb@bb.com, shows, that the email is not using the public key for bb@bb.com and this ends correctly in a blank p7m Mail.

Yes, because you don’t have access the private key for bb, there has to be some way for you to read your own sent messages. They will be additionally protected by the aa private key so you can open them yourself.

But it is encrypted using bb’s public key, and bb will need to enter the own password to view the message.

Thank you for the explanation on the sent e-mail.
But, the Mail-Gateway from bb@bb.com says that the E-mail is not encrypted with the public key for bb@bb.com. In the p7m file are some readable informations. One of it shows only certificate informations from aa@aa.com. So i think that the e-mail was not encrypted with the correct certificate.

Gary, thank you very much for your help.
I cant fix this issue, with Thunderbird bb@bb.com received my mail correct. I dont have a pro license and dont want to buy one. The new Calendar UI in Version 8 forces me to use a different calendar. em client is now complete useless for me.