Allowing path discovery with ICMP on the firewall

Hi,

we have an issue with some of the security devices in our network like email cisco email appliance and symantec proxsy, we can not send emails to some domain through email security appliance and access some sites through proxysg. the support teams recommend us that issue can be fix only by allowing path discovery with ICMP on the firewall. we have fortigate 501E as edge firewall followed by router. Please require your recommendations to do so.