There’s a Mod (or Modification) Simple Machines Forums called the Board Notification Mod that allows Members to activate email notification for new messages.

After installing the Mod (or asking us to install it for you if you are a Forum Hosting follow these customer) instructions:

Members shoud click the Profile tab, then click ‘Notifications and Email’ on the left side and can set the options there. You can experiment with the options but the best settings we found were the following:

Uncheck “Receive reply notification only for the first unread reply.”

Then check the box next to: “When sending notification of a reply to a topic, send the post in the email (but please don’t reply to these emails.)”

And next to where it says “For topics and boards I’ve requested notification on, notify me of,” — select: “Replies and Moderation”

Then each Member should click on each board and click the “Notify” tab next for each board they want to subscribe to email notifications.
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Other Notes:

*To save mail space, the person posting the topic of reply does not receive notification (in our tests anyway) — afterall they don’t need to be notified of something they posted, so the best way to test is to register another user with an alternate email address, login as that test user, and post a test message.

Members can read messages by email, but replying or posting a new
message must be done through the forum (which also serves as an archive
of all communication).

and enjoy the forum hosting service! -OLI