I have a Cisco 871 router called 'ROUTER871'. It is providing Internet and VPN service through a 1M leased line. The Cisco 871 is suppose to replace an aging Fortinet firewall.
Current Internet traffic is normal http, https, ftp and dns. The router is also receiving smtp traffic from 'mail site B' through the Internet. Also, the router send mails out (smtp) to another site called 'mail site A'. So, one site for incoming and another for outgoing.
Everything is fine except that smtp traffic from 'mail site B' is coming in very slow. The mail queue can be very large, as much as ~100 mails. When an attached mail is sent, it slows it even more. If the old Fortinet firewall is used, mails are okay.
I have tried using QoS commands and used the 'service-policy output [policy-map]' command with priority on the WAN interface (FE/4) but it doesn't help.
I am confused why outgoing mails from ROUTER871 has no problems while incoming mails from 'mail site B' is causing slowness.
Does anyone have any comments or ideas?