Supported SMTP headers

You can use the following SMTP headers with Inboxroad:

SMTP Headers:

Date: Indicates the date and time when the message was sent.
From: Specifies the email address of the sender.
To: Specifies the email address of the recipient.
Subject: Contains a brief description or subject of the email.
Message-ID: A unique identifier for the message.
In-Reply-To: References the message to which the current message is a reply.
References: Contains a list of message-ids for previous related messages.
Return-Path: Specifies the email address to which bounce notifications should be sent.

MIME Headers (Common Email Headers):

Content-Type: Describes the type of content within the email (e.g., text/plain for plain text, text/html for HTML, multipart/mixed for mixed content).
Content-Disposition: Defines how the email’s content should be displayed or processed (e.g., inline or as an attachment).
Content-Transfer-Encoding: Specifies how the message body is encoded (e.g., base64, quoted-printable).
Content-ID: A unique identifier for inline content within the email.
Content-Language: Specifies the language of the email content.
Content-Description: Provides a brief description of the email’s content.
MIME-Version: Indicates the MIME version used in the email (e.g., MIME-Version: 1.0).

Additional Headers:

Reply-To: Specifies the email address to which replies should be sent, which may differ from the sender’s address.
CC (Carbon Copy): Specifies email addresses of additional recipients who should receive a copy of the email.
BCC (Blind Carbon Copy): Specifies email addresses of additional recipients who receive a copy of the email but are not visible to other recipients.
X-Headers: Custom headers that can be added for specific purposes. These are not standardized but are sometimes used for various purposes by email clients and servers.
Custom X headers can be added. You might need this to match the feedback to the specific emails / campaigns etc.
We have the following custom X headers that can be used to your liking. They will only show up when they contain information.
header_X-Job
header_X-List
header_X-CAMPAIGN

Updated on 8 January 2026

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