Track and Find Out Whether Your Email Has Been Opened and Read by Your Recipients

The following services (some free, some are not free) lets you track whether your email has been opened and read (some will even let you expire and recall sent email messages):

• MessageTag: now into version 2. Lets you know when and whether emails are opened. Works with Gmail, multiple recipients, and lets you be notified vis SMS.
• DidTheyReadIt? : know when your email was opened, how long it remained opened, where geographically, it was viewed.
• ReadNotify : tells you when email you sent gets read /re-opened /forwarded and a lot more.
• SpyPig : a free email tracking system that is designed to let you know when your friends have read your email, and continues to send an email every time your message is opened by the recipient (up to 5 times). In addition to telling you that your email has been read, SpyPig also tells you the IP address of the recipient.
• BigString : a free 2 GB email service that includes rich secure and certified mail services and lets you password-protect, track, expire or edit sent messages.