How do I retrieve data from an iPhone with a dead memory?

You can't get any information off the phone itself.

You can only get information back from other places – such as iCloud Photos (if you were using it) or backups. In the case of e-mail, that's often set up to keep the master copies on the e-mail provider's servers, in which case, getting back e-mails is just a matter of re-connecting to, or logging in to, your e-mail accounts.

If you were synchronizing data to iCloud, you can see that data by logging into https://www.icloud.com/ .

You can't, unless you backed up the phone before it died. If you didn't backup, the data is gone.