One of my customer was trying to construct OWA url for archive emails in Exchange 2010. Initially even I didn’t know how to construct, so I started playing around with the URL. I created archive mailbox and observed the URL when I browse in OWA.

The URL looked like following:

https://pallav-exchange/owa/?ae=Item&a=Open&t=IPM.Note&id=AMB.RgAAAAAsuyof%2b2qASKYsEw5tZVMLBwCv6rwEn4EDSqL29YcZQoW2AAAAeYgDAACv6rwEn4EDSqL29YcZQoW2AAAAeYhYAAAA.L289Rmlyc3QgT3JnYW5pemF0aW9uL291PUV4Y2hhbmdlIEFkbWluaXN0cmF0aXZlIEdyb3VwIChGWURJQk9IRjIzU1BETFQpL2NuPVJlY2lwaWVudHMvY249UGFsbGF2IFNoYXJtYTdhOS9ndWlkPTRmYjBiNzBlLTdhMWMtNGNlZS05ZjRlLTRmOWM1OGUzMDYxMQ%3d%3d&pspid=_1342538626888_576913107

OOPS! How do I construct this? I browse through several archive mail item and found a pattern.

First Part of the URL is constant for mail Items

This is constant ==> https://pallav-exchange/owa/?ae=Item&a=Open&t=IPM.Note&id

Now the changing part is the value of id parameter, which has three parts separated by .(dot)

Part 1:
AMB

Part 2:
gAAAAAsuyof%2b2qASKYsEw5tZVMLBwCv6rwEn4EDSqL29YcZQoW2AAAAeYgDAACv6rwEn4EDSqL29YcZQoW2AAAAeYhYAAAA

Part 3:
L289Rmlyc3QgT3JnYW5pemF0aW9uL291PUV4Y2hhbmdlIEFkbWluaXN0cmF0aXZlIEdyb3VwIChGWURJQk9IRjIzU1BETFQpL2NuPVJlY2lwaWVudHMvY249UGFsbGF2IFNoYXJtYTdhOS9ndWlkPTRmYjBiNzBlLTdhMWMtNGNlZS05ZjRlLTRmOWM1OGUzMDYxMQ%3d%3d


Explanation:

Part 1:
It is pretty simple, AMB indicates Archive Mail Box, so just prefix.

Part 2:
It is OWA ID of the item. When you get an Item from EWS, it gives you EWS Id of the Item; you can use “ConvertID” method of EWS to convert EWSID to OWA ID.

Part 3:
It is Legacy DN with Mailbox GUID, converted to Base64 string, decoding this will give you

/o=FirstOrganization/ou=Exchange Administrative Group(FYDIBOHF23SPDLT)/cn=Recipients/cn=PallavSharma7a9/guid=4fb0b70e-7a1c-4cee-9f4e-4f9c58e30611


So Archieve MailBox URL = AMB.OWAID.Legacy DN with Mailbox GUID(Converted to base64 string) 

So you can construct URL for archived mail box item with above technique.

Above information is based on my research and there is no documentation on this URL format.