Check your synchronization status in User settings
Check the email address of the contact and the email address in the email message
It happens once and a while the email address saved in Tamago-DB is slightly different than the one used to email the contact through your mailbox. It might contain a dot, hyphen, underscore or a character might have been misread : o and 0, i and l, ...
For example, Gmail ignore dots in the email address :
If someone accidentally adds dots to your address when emailing you, you'll still get that email. For example, if your email is firstname.lastname@example.org, you own all dotted versions of your address: