Strange Behavior of Email Sending

Question

Hi,

I have configured outgoing server and using webmail link for sending emails in vtiger 5.1.0. Outgoing server settings are available to admin only and the settings are:

server-mail.mydomain.com:26
username-account+mydomain.com
password -*******   
authentication-yes

When i login as admin and send email to a user it shows in sent folder as well as the message is delivered to the recipient.

But when i login as another user it shows in sent folder but the message is not delivered to the recipient.

Strangely there is no error. I even had a talk with my web hosting providers, they are saying they are able to send emails through the above details and there might be an issue in vtiger script.What is the issue i am unable to figure out?

Please help me on this.

Thanks