# Custom headers This add-on is available in all AcyMailing versions. If you enable this add-on you can add some custom email headers, that will be added to all sent emails.\ These headers need to be entered in the add-on's options. ### What are email headers? Email headers are invisible information that mail clients use. It contains the email address that sent the email, the receiver, the authentication verification results etc... In Gmail you can see them like this: ![](https://963725040-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-L_D4iy53WFAtKVs3BPA%2F-M_v026hi0mKOuTggARh%2F-M_v2M2hV5Nl5iMD7-h-%2Fheaders%20gmail%201.png?alt=media\&token=1422329c-fc87-4739-8de7-d570a9a50183) It will look like this: ![](https://963725040-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-L_D4iy53WFAtKVs3BPA%2F-M_v026hi0mKOuTggARh%2F-M_v2PHsgCyCSS_IWv9v%2Fheaders%20gmail%202.png?alt=media\&token=f03675f2-cbf9-4365-90b6-33534e765f4b) ### Options ![](https://963725040-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-L_D4iy53WFAtKVs3BPA%2F-M_v026hi0mKOuTggARh%2F-M_v0fWTWVUAO-SIYOYy%2Fcustom%20headers%20options.png?alt=media\&token=62673bb3-0125-4351-bf5c-aaf42b200d71) For each custom header you want to add, you need to enter its name and its value. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.acymailing.com/addons/all-cms-add-ons/custom-headers.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.