Other domain mail's getting failed Server error: '550 Relay not permitted'

Hi,

I am using Sophos XG firewall for the SMTP mails scanning and protection. Here are some basic setup details as:

1. My internal users can send and receive the mail (Internally and outside my domain).

2. We are working on SMTP (Port 25) and POP3 (Port 110) in Outlook (Mail Client) app (connected using Server's Local IP).

3. If my user will be on public internet then he/she can send mail only to internal users (Connected using Public IP). If she will try to send an email to outside the domain then getting below error:

Your message did not reach some or all of the intended recipients.

Subject: test outgoing
Sent: 5/10/2019 11:06 AM

The following recipient(s) cannot be reached:

'deepuverma@outlook.com' on 5/10/2019 11:06 AM
Server error: '550 Relay not permitted'

I know the root cause of this error as SMTP polices is blocking this because my client is also using the default port 25 for SMTP and emails getting a scan and checking protected domain so this domain is not in a protected domain and getting blocked. I am looking for a solution that How Will I resolve this issue without changing my client's default SMTP port 25.