[exim] Sending mail: restricted characters in address

Hi,

I'm trying to send a mail and I'm getting this error:

2009-05-14 20:04:30 [8049] H=xxx.xxx.xxx.xxx.static.snap.net.nz
([xxx.xxx.xxx.xxx]) [xxx.xxx.xxx.xxx]:41801 I=[xxx.xxx.xxx.xxx]:25 F=<xxx@???
> rejected RCPT <+xxx_xx+xxx
+38xxa0.jxxe#pxxxxxxd.com@???>: restricted characters in
address

Is anyone able to point me to the right part of the configuration and
suggest what changes need to be done?

Kind regards,
Samuel