S/MIME encrypted emails – decryption issue

Hi, I use Contact Form 7 with POST SMTP Mailer to send encrypted emails to recipients other than website administrator. All recipients got their S/MIME public certificate added and the administrator’s PGP and S/MIME configuration was cleared (according to this post: Encrypt Mails to non-admin e-mail | ww.wp.xz.cn). The email address provided in each profile corresponds with the one in certificate.

Now, the problem is that all emails are transferred correctly, but encryption does not work fine. Recipient see message starting with “MII” (this part is common for all messages) and followed by gibberish instead of readable text. Also, Outlook does not mark those messages as encrypted – the padlock icon is missing.

Meanwhile, certificates work fine when used in regular client-to-client communication.