<p>I wonder if anyone can shed some light on the following? When replying to mail from one particular colleague (possibly also when emailing him directly, I can't quite remember, but certainly this happens when replying to his messages to me) I *occasionally* receive the following mail delivery error message after sending:</p><p>----- </p><p>Delivery has failed on the enclosed message for the following reasons reported either by the mail delivery system on the mail relay host or by the local TCP/IP transport module:    553 5.1.2 <firstname.lastname@domain.com>... the domain domain.com is not meant to be used in email </p><p>----- </p><p>(I've changed the email address/domain name above but there's nothing unusual about it, i.e. no special characters or anything like that). Initially I thought it was a problem at his end as this issue only occurs when emailing this one particular address but just now I've noticed that the above error message is preceded by the following when I look at it in Pegasus' "Raw view":</p><p>-----</p><p>From: Mail Delivery System <> To: Self Subject: Mail Delivery Failure.</p><p>----- </p><p>I'm wondering whether that might indicate that the problem is internal to Pegasus itself, or perhaps relates to the conversation between Pegasus and my own ISP's SMTP server and has nothing to do with the recipient's email server. Has anyone seen this error message before? I believe it's the first time I've ever encountered it and the odd thing is the issue crops up intermittently, not all the time - sometimes mail gets through successfully and sometimes not, yet the email address is the same. Many thanks in advance for any thoughts! </p>