# additionalHeaders(for:) Provides custom headers to include in the outgoing message. ``` optional func additionalHeaders(for session: MEComposeSession) -> [String : [String]] ``` ## Parameters `session` The session that represents the properties of the message in the compose window. ## Return Value A dictionary that maps header keys to arrays of string values. ## Discussion To add custom headers to an outgoing message, return a dictionary that contains the header key and an array of one or more values. > Note: > MailKit ignores entries in the dictionary for standard headers such as Subject. The following code shows an example of adding two custom headers: one with a single value, and another with multiple values. ```swift func additionalHeaders(for session: MEComposeSession) -> [String : [String]] { // To insert custom headers into a message, return a dictionary with // the key and an array of one or more values. return [ "X-CustomHeader": ["This is a custom header."], "X-CustomColors": ["Red", "Green", "Blue"] ] } ``` The resulting message contains the following headers: ```other X-Customcolors: Red X-Customcolors: Green X-Customcolors: Blue X-Customheader: This is a custom header. ``` > Note: > Header keys in mail messages are case-insensitive, so don’t make assumptions about the capitalization of header. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)