There are many Mac-related web sites, and Apple has many tools posted. Please post the specific URL. (If you're not sure of the URL, then you can Control-Click on the file that was download in Finder, and select Get Info from the pop-up menu, and that display can sometimes provide you with the source URL for the file.)
As for opening specific mail messages causing issues for Mail.app, that can imply a problem with those specific mail messages, or potentially a mailbox corruption.
First, select and issue a mailbox rebuild <Mail.app > Mailbox > Rebuild) for each mailbox involved with the problem message(s). This can rebuild each mailbox, and can sometimes resolve these cases.
If that rebuild fails to resolve this, then one of the easiest ways to resolve that is to log into the web interface from your mail provider (if one is available), and delete the specific messages invoilved. (How you log into the web mail interface (and whether or not your mail provider offers a web mail interface) is specific to the mail provider.)