employs a method of SPAM Protection called Greylisting.
Greylisting (or graylisting) is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate the originating server will, after a delay, try again and, if sufficient time has elapsed, the email will be accepted.
Your sender may see the following in their mail logs indicating Greylisting is in effect
451 This server employs greylisting as a means of reducing spam. Your 
message has been delayed and will be accepted later.

To bypass Greylisting you may whitelist sender email addresses (or domains) from your Webmail Interface at http://webmail.your.domainname/