Slow connection to SMTP service

Hi. I am seeking guidance on trying to track down a problem that has recently appeared. We use a FG500A v4.0,build0291,100824 (MR2 Patch 2) Basically, in the last week or so, opening connections to the SMTP service on our mail server have slowed from being sub second to now taking over 10 seconds to connect. Once connection is established (HELLO message from SMTP server appears) then the actual throughput is fine. Using telnet, I opened a link to port 25 on the server using it' s ip address from the server itself, a machine on the same local network behind the firewall and a remote machine outside the firewall. The connection on the local network and outside the network take ages to connect. The connection on the machine itself, using it' s ip address or 127.0.0.1 happen straight away. POP3 connections to the same server connect straight away and do not have this issue. The mail server has been rebooted several times. I have turned off UTM in the firewall policy to no effect. No changes were done to either firewall or mail server before the problem appeared. As the link on the local mail server happens straight away, I am unsure as to where the issue may lie (firewall or mail server) and really where to start? Any advice welcome! Thank you in advance. Penny