Add custom headers
To add custom headers to the simple MIME message, use CustomHeaders property of TElSimpleMIMEMessage class. Provide custom headers in "Header: Value" form as used in MIME. Remember about space character after colon.
Note, that if you add headers, whose names correspond to the names of "standard" headers (for which the properties exist, such as Subject), the custom header value provided via CustomHeaders will overwrite the value set via standard property.