Generally, digital certificates provide:
Secure authentication: The sender and the recipient validate each other’s public key and therefore, validate each other.
Data integrity: The data exchanged between the sender and recipient is digitally signed. The recipient host validates the digital certificate to determine that the data is not modified.
Messaging uses digital certificate when establishing:
an HTTPS connection to the Apache Web server for the Messaging web interface
SIP-TLS connections
IMAPS connection
LDAPS connection