What happens to messages when the mini is off?

When the iR device is off, messages are held on the Garmin servers (not in the Iridium network). You should be able to retrieve them via a manual message check at any later time. They might also show up via a completely passive check. But this becomes less and less likely as the message grows more stale.

To be clear, the iR devices actively check for messages on power up (at least, I think that's still the case), whenever they send something (a sent track point, an outbound message, a waypoint, or whatever), when you do a manual message check, and (typically) once an hour while powered up. These active checks involve asking the network if there is a message and waiting for a response (either positive or negative). An active check should always retrieve any waiting messages.

Some of the iR devices also do a periodic passive check. Think of this as "listening for a ringing phone". The device listens but does not proactively "ask" about waiting messages. Even for a new message, this is very hit or miss because the device must be listening at the time when the "ring" is sent. It gets worse for older messages because the Garmin servers send the "ring" less and less frequently as the message gets older. Bottom line is that this almost never fetches the message - which is why the devices now actively check once an hour or so.

"Replies" to email messages should work for quite some time. Note that you CANNOT reply directly to the email message using the email system. You must use the link in the email. When you click the link in the email, you will be taken to a web page that displays the location and has a pop-up box where you can send a reply. This works as long as the page is still available. I don't know how long the page remains available, but it's probably longer than the SMS reply information (see below).

Aside: Your iR device ALSO does not have an assigned email address. That thing that looks like an email address - [email][email protected][/email] - is the iR device address for iR-to-iR messages. Nothing to do with email.

Replies to text messages CAN be done direct from the messaging app on your phone. You are correct in that the Mini has no assigned "phone number". The ability to reply to an SMS depends on some extra information that the Garmin server records when the iR device sends the original message TO the phone. The server records the target phone number, the (randomly assigned) outbound number from which the text appears to originate, and the identity of the iR device. When the server later sees the reply, it matches the "reply to" phone number and the number from which the reply originated to get to the iR identity. There is no well-defined time limit to this information. You should be safe for a week. A month would be iffy. I would not expect it to work at 6 months.

Most of us handle this by "priming the pump" at the beginning of each trip. We send a "starting the trip" message to our emergency contacts - both email and phone. That way, we're sure that they all have the ability to reply (even if previous reply information has disappeared from the system).