The following links detail the implementation of EA Disclaimer, S/MIME:
Using Disclaimer, S/MIME Sink
Email Disclaimer
Digital Signature
Email Encryption
Journal and Troubleshooting
Appendix - Set up DomainKeys/DKIM
Appendix - Set up SPF record in DNS server
Online Tutorials:
Add Disclaimer or Signature with Embedded Images in Exchange Server 2007/2010/2013/2016/2019 - Tutorial
Add Digital Signature (S/MIME) to Email in Exchange Server 2007/2010/2013/2016/2019 - Tutorial
Encrypt Email (S/MIME) in Exchange Server 2007/2010/2013/2016/2019 - Tutorial
Add Disclaimer or Signature with Embedded Images in IIS SMTP Service and Exchange Server 2003 - Tutorial
Add Digital Signature (S/MIME) to Email in IIS SMTP Service and Exchange Server 2003 - Tutorial
Encrypt Email (S/MIME) in IIS SMTP Service and Exchange Server 2003 - Tutorial