Hi,
a few days ago I logged into the forum for the first time, did some replies and never got a mail despite having checked the box for getting notifications about new posts in the threads. Today I checked my profile settings and discovered that the required email input was empty. So many users are likely waiting for answers which have been written, but not successfully been sent.
This seems like a serious problem for encouraging new users to continue their participation in discussions. This is made worse as might not even expect that they have to enter their mail address again, because they have a mail address associated with the OSM account they are using to log into this forum.
Is it possible for us to fix this in some way?
Yes, it’s fixable to some extent. Working on it. 
If everything went well, users without configured email address will get a warning to add an email address to their profile at every ‘Email’ and ‘Subscribe to this topic’ position while viewing forum topics.
Thanks for reporting this problem.
Currently 15716 users don’t have an email address set and 2216 have an email address in their profile. Let’s see how this develops.
The forum uses the API 0.6 User Details page to validate the provided credentials, but this page doesn’t return the email address of the user. I don’t know of any other method of retrieving user info.
Another adition: when a user enters his email address as ‘Username’ in the login form then the email profile field is automatically updated in the database.
Whatever you type in the ‘Username’ field in the login form, be it your OSM email address or your OSM screen name, the forum username will always be your OSM screen name.
Thank you for responding to this problem!