Values.MessageHeaderSourceContains the name and value of a message header that you add to an email.
type nonrec t = {name : MessageHeaderName.t;The name of the message header. The message header name has to meet the following criteria: Can contain any printable ASCII character (33 - 126) except for colon (:). Can contain no more than 126 characters.
*)value : MessageHeaderValue.t;The value of the message header. The message header value has to meet the following criteria: Can contain any printable ASCII character. Can contain no more than 995 characters. The combined length of the header name and value must not exceed 996 characters.
*)}