hi guys,

One of our client domain have been detected as spam in SMG. I already add the domain in Reputation --> Good Sender.

Is there any better way to whitelist a domain?

See this

Enabling user defined Good and Bad Senders list (whitelist and blacklist settings) within Symantec Brightmail Gateway version 9.x

How to whitelist outbound traffic when you have email spam scanning enabled for outbound messages

Did you need more help here ?

You have to add ip address to whitelist otherwise smg will block messages . 

SMG first check is ip reputation . 

If there is a problem with the ip address smg will blocks the messages it's not matter domain in the whitelist .. if you want to pass all messages althouth have bad reputation you have to add their ip addresses on the smg .

for example a.com has a bad reputation and you added the whitelist like a.com smg learns domain ip addresses and checks all ip addresses if it's see bad reputation with ip addresses it's will be block but if you added ip addresses on the list it's will be pass ..