Slow access to RBLs?

[Stanier, Alan M]

We are running MailScanner 4.23-11 under RedHat-9

Yesterday email starting running extremely slowly, with messages accumulating
in the Incoming Queue Directory

Messages in /var/adm/maillog showed problems with RBL checking ...

Oct  8 09:01:20 serlinux15 MailScanner[18710]: RBL Check relays.ordb.org timed out and was killed, consecutive failure 4 of 7 
Oct  8 09:01:45 serlinux15 MailScanner[18670]: RBL Check MAPS-RBL+ timed out and was killed, consecutive failure 1 of 7 

I disabled RBL checking, and email started running at full speed again.

It seems infeasible that all the RBLs we check should have developed problems
simultaneously, so the problem is presumably local, but I can't think where.
The network team here assure me they have made no changes which might have such
an affect.

Does anyone have any suggestion?

--------
Alan Stanier
Essex University Information Systems Services
Systems Group