IMAP and NetBIOS do not perfrom well over VPN connections, period. I am in the process of evaluating several VPN products, and that is the one constant. The workaround is to deploy Outlook Web Access and let the users reach email via HTTP. Performance is much, much improved.
vpn-firewall> unset flow tcp-syn-check. Save the configuration. vpn-firewall> save. This resolved the problem for us and should resolve the Outlook & Exchange connectivity issues over VPN even if it is a different VPN device like Cisco ASAs but ofcourse use appropriate commands for those device. Unable to Send Email When Connected to VPN | ExpressVPN Sep 29, 2015 Outlook Disconnected from exchange over VPN Solutions Apr 12, 2013 Limits to using personal folders (.pst) files over LAN and Mar 31, 2020
SMTP - Sending emails while connected to VPN – HMA Support
Solved: Endpoint client on VPN kills Office 365 Outlook/Sk
When Outlook 365 is trying to connect to an Office 365 email account via SSLVPN, I still have to modify the "TAP-Windows Adapter V9" network connection to add the SSLVPN gateway IP to it. My LAN default gateway is my T35's IP of 192.168.16.1, and my SSLVPN is using 192.168.35.1 as its gateway.
Outlook Disconnected from exchange over VPN Solutions Apr 12, 2013 Limits to using personal folders (.pst) files over LAN and Mar 31, 2020