If email delivery fails to one user, does it fail to deliver to all users?

No, when you send an email it is sent to all of the recipients individually.

You should get a "delivery failed" for all of the recipients that failed to get the email for whatever reason. If you do not get a delivery failed notification then you should be able to assume that the email got through.

Depending on the email systems in question though it may be a couple of days before you get the notification as servers may try to resend the email and thus delay the "failed" response.

No, if you have sent let us say to 10 E-Mail addresses, and you get a mail from the Mail Delivery Subsystem saying "Address not found", then the mail is not sent to the mentioned address(es) alone, for others it will be delivered.

I tested this out with couple of my own email ids.

As usually, the answer is it depends.

• On the protocol level, as explained in the other answers, the delivery of email happens separately for each recipient, and failures should be reported for each recipient (though a mail server may choose to send a single failure message for multiple recipients). There is no "back channel" to the sending mail server to cancel mails.
• However, in practice, if delivery fails for one recipient, that may indicate a problem that might impact other recipients, too. And for some of these recipients the mailserver may not send a failure message. So it's possible that other recipients did not receive the mail either.

So you should check why the delivery failed.

• If the problem was specific to the recipient (e.g. mailbox full, address does not exist etc.), then other recipients are generally not affected.
• If there is a problem with the mail itself (such as invalid headers, or attachments that look suspicious to some scanner), or with your mail infrastructure/ISP (such as being on a spam blacklist, or otherwise misconfigured) it is possible that other recipients will reject the mail, too, possibly without telling you.

In particular, if the receiving server considers a message to be spam, they will often not send a failure message (because that would help spammers).