we have 400 to 500 hundred emails, when we concatenate them and put them in mailto: it does not work, browser automatically adds \"...\" in between emails and clicking link
I would suggest a DG for this many people but email addresses are not separated by commas but instead semicolons. I have read some people have issues using semicolons in their mailto line. Try encoding the semicolon as %3B, much like you would encode a space as %20.
For example,
mailto:someone@somewhere.com%3Banother@thatplace.com?subject=THIS%20IS%20AWESOME