Don’t just delete unwanted subscription email messages; unsubscribe from them. This applies even to “friendlies” who automatically add you to their mailing lists.
If you are familiar with the sender and can verify his domain (firstname.lastname@example.org), it is probably safe to click on the unsubscribe link at the bottom of the email message. If you’re not sure, just add the sender to your Blocked Senders list (Junk).
To get rid of the old stuff, sort by sender and then by subject to identify large chunks of email you can delete right now!