Helping people with computers... one answer at a time.
It's easy to delete an email message by mistake. Whether or not it can be recovered depends on many, many things including but not limited to the email service, your email program, how long ago the email was deleted, and even whether or not the courts are involved.
Depending on what email program you use, deleted emails may or may not be really deleted.
Several factors conspire to make recovering a file or message deleted three years ago extremely improbable. Still, there's an extremely slim chance.
Retrieving old emails is only possible if your ISP keeps backups of files from that time. It's unlikely.
In most cases, deleting email really does delete the email beyond your ability to recover it. As always, however, there are exceptions.
Most individuals wouldn't be able to access emails that you've deleted... but that doesn't mean that someone at the ISP wouldn't be able to find the message.
Unless you plan ahead, deleted email is ... well ... deleted. There are some fringe cases where you can recover deleted email, but not easily.
It's easy to overlook important steps if you're attempting to securely and permanently delete an email message. I'll cover what's required.
There are things to enable ahead of time if you would like to save your email history, but if they are not enabled then your emails are gone for good.
Configuring a PC based email program to use IMAP and have it constantly downloading email as a backup is a reasonable way to go.
In practicality, information is deleted when it is deleted, although there are several scenarios where it could be recovered if someone were motivated enough!