Direct Method: Using Message Filters

To set up an auto-reply within Thunderbird itself, you must first create a message template and then apply a filter rule to use that template.

1. Compose the Out of Office Template

• Click the Write button to start a new email.
• Type a subject (e.g., "Out of Office" or "On Vacation") and compose your message in the body.
• Leave the To field blank.
• Go to File > Save As > Template. You can now close this window.

2. Create the Message Filter

• Go to the Tools menu (or the hamburger menu) and select Message Filters.
• Select the specific email account you want the reply to apply to from the "Filters for" dropdown menu.
• Click New to create a fresh filter and give it a name like "Holiday Responder".
• In the "Apply filter when" section, select Getting New Mail and Filter after Junk Classification (this helps prevent replying to spam).
• Select the radio button for Match all messages.
• In the "Perform these actions" section, choose Reply with Template from the dropdown menu.
• Select the template you saved in step 1 from the adjacent dropdown.
• Click OK to save the filter.

3. Activating and Deactivating

• Ensure the checkbox in the Enabled column for your new filter is ticked.
• Important: You must leave Thunderbird open and your computer running for this to work, but it will catch-up when you next open it. 
• When you return, go back to the Message Filters menu and untick the filter to stop the automatic replies.