When users set up an auto-reply message, using either the Vacation Message feature or the auto-reply feature in spamBlocker, they find that not every message sent to their mailbox gets the automatic response. That's correct, not every message sent to you gets an auto-reply, but every *sender* does get an auto-reply. Here's how it works:
The first time any unique sender emails a message to you when you have an auto-reply enabled, they get the response. But each sender only gets one auto-reply per day. If they send a subsequent message within that day, they won't get another auto-reply. If they send another message the next day they'll get another auto-reply from your mailbox. This is by design, and works that way on both the Vacation Message feature, and the automatic reply when you use spamBlocker on High, which goes to senders not yet in your allowed sender list.
This also includes sending messages to yourself. You can only get one auto-reply per day.
There are some very good reasons for this design, mainly to prevent something similar to what is called a "forwarding loop". In this case we call it a bounce loop. What if the sender had an auto-reply on their own mailbox? Then a loop would be started with emails going back and forth as fast as the servers could handle them, until the service melted down and no one could get messages. Or what about spammers that use bogus addresses that can't be replied to? Same thing would happen, your auto-reply would go to a bogus address, that message would bounce back to you with a "user unknown" error, prompting your mailbox to fire off another auto-reply to the server that bounced you, which would probably cause another message back to you. Etc.
So even though it might confuse you when trying to test it, rest assured that every sender gets your message, but no more than once per day. When you create a new Vacation Message, unfortunately you only get to test it once on your own. You'll need another mailbox or the assistance of a friend to test it again. I know this might leave you with a feeling of uncertainty about whether it is really working or not. But it really is. Here's one suggestion that I use myself. Go to My Account and set up another mailbox or two, which takes only a couple of minutes and is free for most subscribers. Then send yourself a message from those and check your auto-reply. Then just delete those extra mailboxes until next time you need one.