Strange issue // Email services slow

Dear All,   I am writing this post from Saudi Arabia. I badly need all your help and advise on this issue which we are experiencing for a long time.   We are using Fortigate 200D Firewall in one of our office. Fortigate 200D Firewall is linked with our corporate Forti Manager device which is located in France. We have configured SITE to SITE VPN to access our corporate services like Intranet, OCS, HelpDesk, and etc.   We are facing slow email connectivity issue on every Sunday morning. Those who are connected through Fortigate  200D Firewall, they are not able to send and receive any emails on SUNDAY ONLY. Everything is normal on other week days. During the time of incident, Internet is working perfectly. I can access all other websites without any trouble. There is no schedule services running on that time.   Before we suspect that there might be an issue with VPN traffic to access the local file server. Last week, we have disabled the VPN rule to access the file and check. We faced the same problem again so now it is confirmed that there is no problem with VPN traffic. We have installed the latest firmware too. But, this issue keeps reoccuring again and again on every Sunday morning.   To be honest, I am almost helpless. My corporate team has tried their best to resolve this but no luck. I am here now to seek help from the experts.   Please help me on this and advice me the necessary workaround to identify this issue.   Thanks in advance   Best Regards, Barakath