Why did my email get sent to old contact list email address?

Yes it is possible that you will send a email to the email address which was used by the one of your friend or any other person before because you have the particular email address on your contact list.

Mainly this problem come because of the Auto Complete List option provided by the email provider or email program.

An auto complete list option is an option at which you can select the email address only with press starting letter of the first name of the email recipient.

But when auto complete list arrange the email contacts to show they use mainly those contact which you have saved on your contact list of address book.

Now a days because internet email become the first medium of communicate with each other so email provider and email program always update their setting so auto complete list settings also been changed now.

Now a days when you want to select the particular email contact from auto complete list the email contact have been shown on the auto complete list is the combination of contact list and the email address at which you have sent emails before.

So it may be possible that you have sent the email to the particular wrong email address because you have sent email before to the email address. And now when you wrote the starting letter in the "To:" field auto complete list given you the email address of your friend at which you have sent emails before.