"but I am the only person at the moment who is using this exchange server.
PLUS these ACKs are coming from just 3 IPs 24/7!"
So the only traffic outbound from pfsense is this exchange server, there is NO clients behind pfsense?
Also the ips your seeing are NOT on the list from godaddy for their CRLs - but yes crl is a FQDN, and its served up from a CDN so its IP will change I would assume.
;; QUESTION SECTION:
;crl.godaddy.com. IN A
;; ANSWER SECTION:
crl.godaddy.com. 855 IN CNAME gdcrl.godaddy.com.akadns.net.
gdcrl.godaddy.com.akadns.net. 12 IN A 50.63.243.228
So its quite possible that IP changes..
As to the oscp
;; QUESTION SECTION:
;ocsp.godaddy.com. IN A
;; ANSWER SECTION:
ocsp.godaddy.com. 1647 IN CNAME ocsp.godaddy.com.akadns.net.
ocsp.godaddy.com.akadns.net. 31 IN A 72.167.18.239
I really would watch a full sniff to see if your sending out traffic to these IPs - which don't really seem to be CRL or OSCP.