Gmail – How to set up vacation autoresponder with non-Gmail email in Gmail

I worked it out.

In the original email that Gmail sent out to the external address in order to "verify" it there was the following section:

If you accidentally clicked the link, but you do not want to allow xxx@yyy.com to send messages using your address, click this link to cancel this verification: https://mail.google.com/mail/blahblahblahblahblah

I found it just by grepping his maildir, clicked the link and it said he can no longer send using his work address.

I realise you can just spoof the "from" headers willy-nilly, but you have to be technically savvy enough to know that this is possible and 99% of my users are not, including this one.

Yes, it's possible. Unfortunately, unlike the vacation autoresponder which only sends the message once to the sender, you'll end up sending the message every time you receive a message.

First, you need to turn on the Canned Responses lab and set one up.

(To create a canned response, start by composing a new message, then in the context menu of the compose window choose Canned responses → New canned response….)

Once you've got your canned response, now you need a filter.

Create a filter with the trigger from:john@example.com and choose as the action Send canned response: and choose the canned response you just created. You may want to make the filter more specific and/or choose other things to do with the message. (There are lots of posts here that cover Gmail and filters. Look under the gmail-filters tag.)

Now, whenever a message matches your criteria, a response with your canned text will be sent immediately back. As I mentioned, however, this will happen with every message that trips your filter.