Default limit for the number of IMAP & POP3 connections per IP address is 4.
Probably outlook opens 2 connections to IMAP server per account.
You can change this limit inside the
/etc/courier-imap/pop3d
/etc/courier-imap/imapd
configuration file, option is question is:
MAXPERIP=4
Cambiarlo a 8 o mas y restaurar el servicio.
/sbin/service courier-imap stop
/sbin/service courier-imap start