What is the most likely cause when an analyst cannot open an encrypted email?

The most likely cause when an analyst cannot open an encrypted email is that the S/MIME plug-in is not enabled. S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for public key encryption and signing of e-mail messages. When an email is encrypted using S/MIME, the recipient's email client must have the appropriate plug-in or support enabled to properly decrypt and read the email. If this plug-in is not enabled, the email client won't be able to process the encrypted content, resulting in the inability to open the email.

The other options address different aspects of email security and protocols. An expired SSL certificate would hinder secure communication, but emails might still be readable. Not implementing Secure IMAP does not directly relate to the ability to open encrypted emails; rather, it affects the security of email retrieval over IMAP. Likewise, POP3S not being supported would primarily affect emails retrieved using the POP3 protocol, not necessarily the ability to open encrypted emails once they are sent and received. Therefore, the absence of the S/MIME plug-in is the direct reason for the inability to open the encrypted email.