[Exim] Problem receiving mail

Hello,


My configuration
Exim 4.05
linux 2.4.20

When one of my correspondents try to send me an email (is mailer is lotus notes), i sometime got this error in exim_main.log:

SMTP command timeout on conection from [xxx.xxx.xxx.xxx]


The only thing i've noticed, when this occurs, is that a host command leads to host xxx.xxx.xxx.xxx not found (i think their DNS is down).

Is this DNS problem the raison why i can't receive their mails ?
I tried to put the IP in a relay_from_hosts macro, and accept it in the ACL list, but it's not better.

Can someone give me a clue of how i can fix the problem ?

Thanks

--
Pascal