I have used for over 10 years now MSGTAG free edition on my workhorse OE Email account with not a single problem and I have almost always received a message saying that the recipient has opened my mail from a large variety of highly official and business sources to friends.
With my Gmail account I have just discovered Bananatag for Google Chrome as a simple extension
With a few simple adjustments to the settings, it works perfectly with no trouble whatsoever. Every new Email has the Bananatag marker and tracker checkbox included and a receipt message is received immediately the recipient opens the sent mail. Brilliant, it is extremely user friendly and the support people can be contacted easily and are prompt in replying.
With the free version, 5 Emails per day only are allowed, but for myself and many more casual Email senders, that is more than adequate.
Edited by Orange Blossom, 16 October 2013 - 08:35 PM.
Moved to more appropriate forum. ~ OB