Why the mac can not access external traffic to other port except 80, 443, 22

The Delay is due to postfix doing a reverse lookup on the client IP and doing another lookup on that result to see if they get back to the IP.

if you are using a consumer connection to the internet most likely the first test will pass. but the second test will cause the problem.

you can verify this on the server itself.

nslookup <your client IP>

returns something like 80-41-90-213.dsl.iwx.uk.net

but on reversing that

nslookup 80-41-90-213.dsl.iwx.uk.net 

you will probably get SERVFAIL after a delay.

it is that delay that is showing up when you connect to the server. To find out which one go through each server in /etc/resolve.conf

nameserver x.x.x.x
nameserver y.y.y.y
nameserver z.z.z.z

test them with

nslookup 80-41-90-213.dsl.iwx.uk.net x.x.x.x

One or more will show a delay before they come back to the command prompt.

Once you've found which DNS causes the big SERVFAIL delay, replace it with another. i.e. Googles 8.8.8.8 and 8.8.4.4 currently return relatively quickly.

It sounds like what you're trying to do is called Split Horizon DNS, and it can be quite tricky. That said, it sounds like you have quite a bit of it already set up but that the internal DNS is not authoritative for your domain.

If you have not already done so, you need to modify your internal DNS to create a primary zone for example.com. Then, when you query for your domain, the DNS should not look outside for an address.