You don't need a new account, noclue. Subdomain addressing is part of every FastMail account. See the following:
https://www.fastmail.com/help/receive/addressing.html
Say your account is foo(AT)fastmail.fm. If you set up the subdomain address
subscribe(AT)foo.fastmail.fm, then all mail sent to that address will be put into your FastMail "subscribe" folder--if you have one--automatically. If you don't have a "subscribe" folder, all mail sent to that subdomain address will be put into your FastMail Inbox--automatically.
If you wish, you can use any number of subdomain addresses. For example:
newsletter1(AT)foo.fastmail.fm
newsletter2(AT)foo.fastmail.fm
and so forth
Oh, I see that Berenburger beat me to the punch about not needing a new account.