# MEExtension A type that provides objects for manipulating email messages, such as performing actions on messages or blocking content when users view messages. ``` @MainActor protocol MEExtension : NSObjectProtocol ``` ## Overview To implement an app extension, you provide an object that conforms to the [`MEExtension`](/documentation/MailKit/MEExtension) protocol. The key of your extension’s `Info.plist` defines the capabilities that you support, as follows: ```plist NSExtensionAttributes MEExtensionCapabilities MEContentBlocker MEMessageActionHandler MEComposeSessionHandler MEMessageSecurityHandler ``` For each capability that your extension defines, you provide an object that implements the capability. MailKit uses the following methods to request the object for a given capability. The capability also specifies a protocol that the handler implements. |Capability Key |Method |Protocol | |--------------------------|-----------------------------------------------------------------------------------------|--------------------------------------------------------------------------| |`MEContentBlocker` |``doc://com.apple.mailkit/documentation/MailKit/MEExtension/handler(for:)`` |``doc://com.apple.mailkit/documentation/MailKit/MEContentBlocker`` | |`MEMessageActionHandler` |``doc://com.apple.mailkit/documentation/MailKit/MEExtension/handlerForMessageActions()`` |``doc://com.apple.mailkit/documentation/MailKit/MEMessageActionHandler`` | |`MEComposeSessionHandler` |``doc://com.apple.mailkit/documentation/MailKit/MEExtension/handlerForContentBlocker()`` |``doc://com.apple.mailkit/documentation/MailKit/MEComposeSessionHandler`` | |`MEMessageSecurityHandler`|``doc://com.apple.mailkit/documentation/MailKit/MEExtension/handlerForMessageSecurity()``|``doc://com.apple.mailkit/documentation/MailKit/MEMessageSecurityHandler``| ## Topics ### Blocking Content [`- handlerForContentBlocker`](/documentation/MailKit/MEExtension/handlerForContentBlocker()) Returns an object that provides rules that the message viewer uses to block content. ### Performing Actions on Messages [`- handlerForMessageActions`](/documentation/MailKit/MEExtension/handlerForMessageActions()) Returns an object that performs actions on mail messages as the system downloads them. ### Enhancing the Compose Window [`- handlerForComposeSession:`](/documentation/MailKit/MEExtension/handler(for:)) Returns an object that participates in the composition of a mail message. ### Encrypting and Signing Messages [`- handlerForMessageSecurity`](/documentation/MailKit/MEExtension/handlerForMessageSecurity()) Returns an object that applies security measures such as encryption and digital signatures to messages. ## Relationships ### Inherits From [`NSObjectProtocol`](/documentation/ObjectiveC/NSObjectProtocol) --- 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)