Alias
Sometime ago, I discovered Hotmail/Outlook provided a free 'alias' service. An alias is where you can make a new 'ghost email' address which actually works, and anything sent to that ghost email will actually be sent to your inbox. This means you can create one email address to work with multiple new RuneScape accounts.
Read more here.
What are the benefits?
Validate multiple RS accounts quickly from one inbox.
No additional sign-up.
Validating new accounts with a Microsoft email is undoubtedly less suspicious than some throwaway email service.
If you've somehow lost an old, old email, you could perhaps traffic any new mail to that old account to your current one.
What are the drawbacks?
You can only add 10 aliases per account each year. But you can create as many new email accounts as you desire.
Using a throwaway email service will be quicker and less of a hassle to deal with.
How do I set one up?
Login to your Microsoft email account and go to your inbox.
Click into settings and then options.
Navigate to Mail > Accounts > Connected accounts.
Under "Email aliases" click on manage.
Follow Microsoft's user guide.
Due to my religious hacking escapade with Warrock back in the day (thanks leaked premium trainer :D), I have a lot of Hotmail emails. If I were to list them all up, I could possibly set up over 100 aliases. However, It'd be easier to make new Microsoft email accounts than trying to remember the passwords for my stupidly old emails from well over 10 years ago.