PHP not able to send email

Hi, Recently, I have found that I have had a problem sending emails with particular accounts on my RedHat Linux 6.2 server using Sendmail-8.9.3. There are two examples of my inability to send email and I think they are related. I have a PHP script used to send email and the user rights of the file containing the PHP script belong to a user called "webuser" (same as the user of my Apache server). When I use the script to send email, I noticed that the email was not sent and when I checked the "mail" logs, I found the following: Dec 17 12:59:16 ns sendmail[14686]: NOQUEUE: SYSERR(webuser): queuename: Cannot create "qfMAA14686" in "/var/spool/mqueue" When I telnet into the machine, log on as "webuser" and try to use Pine to send email , I noticed the following error occurs. [Mail not sent. Sending error: 451 queuename: Cannot create "qfQAA15157] However I was able to resolve the problem in Pine by setting the "mail.visors.com.hk" as my outgoing mail server. Something that I never had to do before yet still successful in sending out email. The same thing happened to another account also, but these accounts were just find previously. I have been reported to have had a recent telnet attack , but I am not sure if this could be the cause of it. Most grateful if someone can shed some light on this for me? Thanks Peter