client cannot send mail

[-keevill-]

I have a client who is unable to send mail to me. Other people have no
problem He can also send mail to me using his Hotmail account. He can send
mail from the company account to other mailboxes , he just has this problem
with my company mail account.
what can the problem be here?
Here is the header on the returned mail.
Is there an error my side or his side??

while talking to mail.domain1.com.:
MAIL (e-mail address removed) <mailto:[email protected]>>
<<< 553 (e-mail address removed) <mailto:[email protected]>... Access denied,name
domain2.com unknown to
DNS
501 5.6.0 Data format error
.... while talking to mail.domain1.com.:
MAIL (e-mail address removed) <mailto:[email protected]>

<<< 553 (e-mail address removed) <mailto:[email protected]>... Access denied,name
domain2.com unknown to
DNS