The MailMessage.Headers property allows an application to access the headers collection for the message. While this collection is read-only (a new collection can not be set), custom headers can be added to or deleted from this collection. Any custom headers added will be included when the System.Net.Mail.MailMessage instance is sent. Before a message is sent, only headers specifically added to this collection in the MailMessage.Headers property are included in the collection. After the System.Net.Mail.MailMessage instance is sent, the MailMessage.Headers property will also include headers that are set using the associated properties of the System.Net.Mail.MailMessage class or parameters passed when a erload:System.Net.Mail.MailMessage.#ctor is used to initialize a System.Net.Mail.MailMessage object.

If some mail headers are malformed, they could cause the email message to become corrupted. So any mail header in the headers collection that can be set using a property on the System.Net.Mail.MailMessage class should only be set using the System.Net.Mail.MailMessage class property or as a parameter passed when a erload:System.Net.Mail.MailMessage.#ctor initializes a System.Net.Mail.MailMessage object. The following list of mail headers should not be added using the MailMessage.Headers property and any values set for these headers using the MailMessage.Headers property will be discarded or overwritten when the message is sent:

• Bcc

• Cc

• Content-ID

• Content-Location

• Content-Transfer-Encoding

• Content-Type

• Date

• From

• Importance

• MIME-Version

• Priority

• Reply-To

• Sender

• To

• X-Priority

If the application does not specify an X-Sender header using the MailMessage.Headers property, the System.Net.Mail.MailMessage class will create one when the message is sent.

The sender, recipient, subject, and body of an e-mail message may be specified as parameters when a erload:System.Net.Mail.MailMessage.#ctor is used to initialize a System.Net.Mail.MailMessage object. These parameters may also be set or accessed using properties on the System.Net.Mail.MailMessage object.

The primary mail message headers and elements for the message may be set using the following properties of the System.Net.Mail.MailMessage class.

Attachments	MailMessage.Attachments
Blind carbon copies (BCC)	MailMessage.Bcc
Carbon copies (CC)	MailMessage.CC
Content-Type	MailMessage.BodyEncoding
Encoding for custom headers	MailMessage.HeadersEncoding
Message body	MailMessage.Body
Priority	MailMessage.Priority
Recipient	MailMessage.To
Reply-To	MailMessage.ReplyToList
Sender	MailMessage.From
Subject	MailMessage.Subject

Namespace: System.Net.Mail
Assembly: System (in System.dll)
Assembly Versions: 2.0.0.0, 4.0.0.0
Since: .NET 2.0