There was a previous issue in 8.5.1 FP4 in which the error would appear whether the user created, forwarded or replied to an email:
http://www-01.ibm.com/support/docview.wss?uid=swg21454229
A troubleshooting step is to disable referral checking:
http://www-01.ibm.com/support/docview.wss?uid=swg21412267
While you are on 9.0.1 and not 8.5.1, there may be something similar happening.