An increasing number of clients are looking for e-mail scanning and archiving systems that can handle S/MIME-encrypted messages. E-mail encryption methods are increasingly popular these days to protect e-mails both within the enterprise and externally. When you encrypt a message, it is no longer searchable, and users can no longer retrieve past email based upon message text keyword searches. Many of my customers are looking to an archiver that can store and search messages that can be indexed and retrieved at a later date.”]
Source: https://www.csoonline.com/article/2630482/the-trouble-with-s-mime-e-mail-encryption.html