We have an IPSEC site-to-site VPN between a Cisco 850 router and a Cisco VPN Concentrator.
Clients are able to browse the Internet and receive e-mail, carry out DNS lookups etc but are unable to send e-mails with attachments. E-Mails without attachments are going OK, but anything with an attachment times out.
The VPN is set to tunnel all traffic, with the hosts specified on the Router and the Concentrator. I can see the POP3 and SMTP traffic on our Firewall logs when the clients attempt to download and send e-mail.
I have tried changing the MTU size on the router for the Dialer interface from 1500 to 1458 (as recommended in a number of Cisco documents) and to 1400 (to see if it made any diference at all), but there is no change, e-mails with attachments fail.
The ADSL speed is only 128k (not fantastic) but the same site had ISDN (so 64K) previously and this was sending fine. Using a different Mail client makes no diference, and trying to upload a file to a web-based mail system also fails.
After reading many articles on t'interweb it looks to be related to the MTU size, but having changed this I am now at a loss.
Has anybody else encountered any similar issues?