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
Its a bad practice, its barely possible. Try to rethink your script.
RFC 1783 (URLs) says that a mailto: URL takes an RFC822 address specification. RFC822 is obsoleted by RFC2822; and RFC2822 says that an address specification can include a mailbox-list, which is a comma-separated list of mailboxen. I conclude that what you are trying to do is permitted.
However UA support for "mailto:" is at best patchy; the recommendation is to avoid it completely, or failing that restrict yourself to specifying a single address.
Attempting to include a subject, in particular, will definitely not work in all UAs. Also note that
It is much better to use a formmail script. That will give you the kind of versatility you appear to want, while completely sidestepping the mailto: URL scheme, the deficient browser support that it implies, and the question of whether any email UA is configured in the first place.
credits: http://bytes.com/topic/html-css/answers/496355-multiple-addresses-mailto
Depending on your use case, it might be possible to send an email from the browser like this, but it relies on the user having set up the protocol handler for mailto properly.
For a limited number of users e.g. a few site administrators this could be a good solution. As you can control the set up of the client machine and make sure that it works.
So if you created a page with this link
mailto:me@example.com?bcc=someone_else@example.com
mailto:someone@example.com?subject=This%20is%20the%20subject&bcc=someone_else@example.com&body=This%20is%20the%20body
Cut and paste in to the address bar to test if this will work on your machine.
I wouldn't recommend this as a robust solution for any good software product, you should use a server based solution. But if you need something to help with internal systems, or just for a personal site, then this will take minutes and cause fewer headaches then server based solutions.
There are many many options for sending email, I think this one is simple, quick and flexible, but it will not work on all machines, maybe only 70%???
One last thing, usually if you are sending emails to multiple recipients send it to yourself and use the Blind Carbon Copy field (bcc) This means that your not sharing your customers' email addresses with all of your other customers.
tl;dr
To summarise, if you can test/troubleshoot every single client machine that will use the email link, then consider using a complicated mailto link. Otherwise go for a server side email.
And use the Bcc field for mass emails.
The maxlength 2048 (if you are using Internet Explorer). I think you might what to reconsider you solution.
Refrence here
If you need to email more than one (but not hundreds), the correct form should not have spaces but should have semicolons (especially if the users will likely use Outlook).
<a href='mailto:one@email.com;two@email.com;three@email.com'>Contact us</a>
If you want to automatically include a subject line add "?subject=This is the subject"
<a href='mailto:one@email.com;two@email.com;three@email.com?subject=Webpage contact'>Contact us</a>
You should create a Distribution List and add all of these emails to this list. http://support.microsoft.com/kb/284292
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