Hi Eugen,

You can attach a "delivery" and/or "read" receipt request to every email you send.
In Oulook 2010 this can be found @:
open a new mail-goto options(on the ribbon at the top of the new mail) there you will see two boxes just tick them and you will get a receipt mailed to you every time the sent mail is received/read.
So to keep track of who has received you'r mails easily you then need to create a folder in outlook called reciepts then create a rule to move all received receipts into the receipt folder. Then in the folder you will have an easy to see list of who has received and read you'r mails.
Perhaps switching to conversation view@
in outlook 2010 choose view from the ribbon and select "show as conversation" is an option for you as well.

If you can't create rules or find any of the things I mentioned then let me know and we can go through it blow for blow