If you are unsure if firewall is causing issues, use tcpdump to troubleshoot.
For eg, to check outbound emails
tcpdump -i any -nn src host My.ip.add.ress and dst port 25
Check if the connection is established or continuously sending SYN requests.
Also you can hping3 to trace, if it is getting blocked on server side.
hping3 -T -S -p 25 des.ti.na.tion
Ctrl Z to increase hop. It works like traceroute. If firewall is blocking, you will not get a TTL timeout reply.