Asking for a 'Return Receipt' and your recipient seeing it or even choosing to acknowedge it is not the same.

Some servers simply do not allow return receipts so the emails will arrive without that facility. Some people will receive a 'return receipt' but choose to ignore it.

Useful links: http://kb.mozillazine.org/Figuring_out_whether_the_recipient_read_your_message

https://support.mozilla.org/en-US/questions/1149090

https://productforums.google.com/forum/#!topic/gmail/4EQIyP9hhYk

Thanks a lot for answering me. I knew there was a lot of problems using it but with all this problem if I decide to use it anyway will it functioning with Mailing list? Can't se any info about this in all the links you send me?

I have been using http://www.readnotify.com/readnotify/rn5help.asp for over 10 years with other mailprograms - will it be better to use this together with Thunderbird and do you know of some one that use this 2 software together and are satisfied?