User accounts already created automatically to get "login retrieval message" possibility

Hello altogether,
first of all, i am rather new to Drupal, but have to say that I am amazed about all the possibilities it brings and how easy it is to set up and maintain.

About my problem: We have a mailing list, which I would like to implement into a forum-page. I condfigured mailhandler and listhandler, so that everything works fine so far and all the mails sent to the list go to the right forum.

The only thing is: whenever somebody writes a message to the list, it gets published to the forum and listhandler creates a new account for the author. so far so good. but if this person wants to sign up on my page to view the other forums apart from the mailing list, he cannot sign-in with his email-adress, because this one is already used for the listhandler-account.

is there a possibility to soemhow get rid of this problem? i was searching the forums but did not find anything apart from some other topics with the same issue, but no solution so far.

i could think of some solution like when listhandler creates the accounts, to store some variable in the database, that this is a listhandler-created account. when a user trys to sign up with an email-adress that matches with a listhandler-account in the database, it should not show the "email already in use"-error, but just send him the activation link/new password and update the database entry with the new information provided. would something like this be possible?

i tried to do realize it myself, but my php-skills are too poor and i could not figure out how to manage it. somebody who would like to help?
thanks so far,
paul