Revert "Update mailparse requirement from 0.15 to 0.16 (#366)"

This reverts commit 67737fe.

Reverting because a test case started failing. Needs investigation.

Cargo.toml

@@ -70,7 +70,7 @@ io-extras = "0.18.0"
 # For DDS Security:
 serde-xml-rs = { version = "0.6" , optional = true } # for reading spec-mandated XML config files
 glob = { version = "0.3" , optional = true } # for reading spec-mandated XML config files
-mailparse = { version = "0.16" , optional = true } # for reading S/MIME-encoded (XML) config files
+mailparse = { version = "0.15" , optional = true } # for reading S/MIME-encoded (XML) config files
 x509-certificate = { version = "0.24" , optional = true } # for configuration certificates
 x509-cert = { version = "0.2" , optional = true }
 tempfile = { version = "3" , optional = true } # for calling external openssl command. Remove when no longer used.