I suggest you do the following, noclue:
(1) Sign up for a free Zoho Mail account.
(2) Have your newsletters sent to your Zoho Mail address.
(3) See if the newsletters arrive in your Zoho Mail Inbox. If they do, you can have them POP-fetched by your FastMail account. (You don't have to have FastMail retrieve them, of course.)
(4) If you want your FastMail account to fetch the newsletters, you must first go to your Zoho Mail account to "enable" POP access to the account. To enable POP access, see the following:
https://www.zoho.com/mail/help/pop-access.html
(5) After you have enabled POP access to your Zoho Mail account, go to your FastMail account and configure a POP Link for your Zoho Mail account by using the following settings:
Incoming Server Name: pop.zoho.com
Port: 995
Require SSL: Yes
Username:
username@zoho.com
Also, be sure to "leave" messages in your Zoho Mail account (that is, leave them on the Zoho Mail server).
Remember, steps 4 and 5 above are "optional." You can choose "not" to have FastMail retrieve your newsletters. I included steps 4 and 5 solely to save you from having to log in to your Zoho Mail account to read your newsletters.
Should you have any questions, feel free to ask.