Php imap_mail email bounce back if email address not in address book

Php imap_mail email bounce back if email address not in address book

I am using imap_mail to send the emails.
We have came across the case when user has typed email address in upper
case. For example email address is test@test.com and user typed
Test@test.com.
We get following bounce back message
From: Mail Delivery System [mailto:MAILER-DAEMON@xxxxx.xxxx.com] To: Test
Subject: Undeliverable: subject text
Your message did not reach some or all of the intended recipients.
Subject: subject text
Sent: 9/4/2013 1:25 PM
The following recipient(s) cannot be reached:
Test on 9/4/2013 1:25 PM
The e-mail account does not exist at the organization this message
was sent to.
Check the e-mail address, or contact the recipient directly to
find out the correct address.
< #5.1.1 X-Postfix; unknown user: "Test">