There is needs to change Symantec Messaging Gateway(SMG) outbound port to 465 port.
About 465 Port：
IANA has reassigned a new service to this port, and it should no longer be used for SMTP communications. However, because it was once recognized by IANA as valid, there may be legacy systems that are only capable of using this connection method. As a result, Mailgun supports SSL connections via port 465. Typically, you will use this port only if your application demands it. A quick Google search, and you'll find many consumer ISP articles that suggest port 465 as the recommended setup. It is not RFC compliant.
It can be changed by navigate to SMTP -> outbound and modify the port in "Outbound Mail Settings" to 465
Clicking save button and wait to take effect,after that SMG can send email from port 465.
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