Each mail sent with mailster is technically a multi part message in MIME format. It contains three sections:

  1. Header
  2. Plain Text Part
  3. HTML Part

Next to some default header values Mailster adds these header properties to each outgoing mail

X-Mailer: Mailster 2.3.1 (5.2.22)
X-Mailster: 3b4160f015ca9e739b8ca942dfe9e8b1X-Mailster-Campaign: 28
X-Mailster-ID: be07c931957062f38291a039d49ccf2f

X-Mailer: Mailster X.X.X (Y.Y.Y)

This identifies that Mailster is the sender of this message. It contains the version number of Mailster (X.X.X) as well the version number of the used PHPMailer (Y.Y.Y)

X-Mailster : [md5hash]

This contains the hash of the subscriber stored in your database. It’s unique to each Mailster installation.

X-Mailster-Campaign: [ID]

This contains the ID of the campaign. It’s the same as the post ID from any of your posts.

X-Mailster-ID: [md5hash]

A unique hash generated on plugin activation. This is used for bounce handling to identify the correct installation. It’s unique to each Mailster installation.

Some headers are required by email services like Gmail or Yahoo.

List-Unsubscribe: <https://example.com/mailster/unsubscribe/28/>
Precedence: bulk

List-Unsubscribe

Contains a link to your unsubscribe age and makes it easy for users to opt out. If you have automatic bounce handling enabled a mailto: address will also be included.

Precedence: bulk

A required header if you send bulk mails. Major email providers will notice if you remove that and more likly mark your message as spam.

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