As communities grow, older accounts are beginning to bounce email notifications as the email address no longer exists.
I can see an option within joomla to require a password reset, but strangely none for email addresses.
This would be a great option to have for admins to have on the front end. A checkbox or button.

At the moment I'm just sending messages to inboxes that seem to get ignored, so looking for something more workable. There is already a forced avatar plugin, I'm guessing a forced email update might work along a similar line?

Not that simple. Those are plugins that force to fill required fields after registration. Maybe you can try with JS Force Fields plugin and delete user email in back-end. See if that is the way to force user to fill missing email field.
On the other hand, there is periodic password change plugin for Joomla here:
macrotoneconsulting.co.uk/file/122-passw...-system-plugin-0-1-7
You can try to modify that one to include email too. That would be more consistent automatic solution.

I did consider the force fields plugin, but it seems you cannot delete an email address, if you put a blank in and press save, an error message that an email address is required... doh!
I'm not sure a periodical email change would be a good thing, I don't want to annoy users, and some of the emails that have expired are actually more recent ones. Even if I enforce a yearly change, its not going to catch the more recent members.

Main thing I'm trying to do is avoid getting placed on spam filters, and also encourage people to log in more (because of a notification receipt)

Here is a sort of a solution:
extensions.joomla.org/extension/force-password-change/
You can reset last visit date to zeros and force to update email instead of password. Extension is not bullet proof since user can hit Save and save same email. You can add additional logic to force email change but that implementation need additional edits on JomSocial registration.

This feature is actually built into joomla now.. also I see the extention is from 2009. I'm not sure if I would be comfortable using it. I'm going to trawl through the joomla site at some point and see if I can find anything a bit more user friendly.
To me this is a glaring omission by joomla =/