answer from unknown 206 ?

It MIGHT be that this address will resolve itself after the unit has been on (and with a clear sky view so it can reach the Iridium network) for a while. Or not.

There is a change in contact address resolution in progress. Off hand, I don't recall if the 66i is included in the initial round of changes or not.

Particularly if the email recipient is one of the contacts on the account (in other words, if the email recipient is listed as a contact at explore.garmin.com), you may need a sync. In general, you should sync after ANY change to the account (not just changes to contacts). If the lack of a timely sync is the problem, I doubt that the unknown address will resolve after the sync. But I could be wrong. Note that the lack of a timely sync is the thing that the new address resolution scheme is supposed to fix.

When you send a message to a registered contact (one which appears in the contact list for your account at explore.garmin.com), the message is actually addressed using a short code number NOT the actual address (email address, phone number, or whatever). On the server side, the short code number is translated to the corresponding real address. The server then sends the message on to the real recipient. 

I BELIEVE that the same thing happens when the recipient replies (for example, via email). The email message goes to the Garmin servers. When the server detects a match between the email address and a contact on the account, it converts the email address to a short code number. The short code number is what gets sent to the device. 

The point of this complicated exercise is to shorten the satellite messages to leave more room for text.

In order for this to work correctly, the device and the server MUST AGREE on the meaning of the short code numbers. This is one of the things that sync is supposed to guarantee. This scheme is notoriously fragile, which is why a new method is being rolled out.

Yes, you CAN send to any email or SMS address. Instead of choosing from a contact on the list shown on the device, type the explicit recipient address. For SMS text messages, the number must include the country code (1 for US and Canada - varies for other countries). The phone number which follows the country code must be in the proper format for that country.

The email recipient can reply via the web-based reply form (link will be in the email itself). This requires Internet access. The SMS recipient can reply directly from the text messaging app on the phone. This requires cell service.

the question is what exactly is a registered account probably.

I log into the explore website in an account belonging to a 66i.

I edit or enter under contects new contacts. Including Name, phone number (for sms) and email.

I presse done.

Then I *sync* the device with the server by using explore app on the phone.

The contacts are now in explore on the website, but are also in the explore app and also on the 66i. (sync successful)

Now I go to messages, either on 66i or associated phone with explore.

I send an email to one of those recipients. Einther directly from the 66i or from the explore on the phone.

The recipient answers by clicking on the link included in the mail.

On computer this will open it in the default browser. Recipient goes into the msg field and types answer there and send.

This will arrive as message from unknown 206 on my 66i (explore at phone)

The same will happen when the recipient selects the link on the phone, explore site opens, he enters an answer and send.

Also this will arrive also as from unknown 206.

So is the meaning, that any email recipient I will ever communicate with the 66i needs some registration process with garmin? That I can not communicate any more with recipients outside of garmin world?

No, what it means is that the contact resolution is broken again. This is a recurring problem with the current mechanism. The only thing I can suggest is that you open a support ticket.

OK, will do. Have opened number of tickets, no idea if this will give any kind of answer one day. Working with trackers now since 2014, I have so far never received some answers to the tickets.

We will see.

currently definitely only recognised correspondence partners are those who have own account within garmin wolrd.

Have ticket on this subject. They replied that the problem is known, but no perspective when this can be solved.

OK, support told me to make factory reset, start all again. Ans yes, it was fixed.

But with other 66i, which I also reseted, the problem is persisting.

It looks like they have to fix each 66i in the world one by one. :-(