Check Email Authenticity
This is what you could to do see if the email is coming from PostmanSG.
If you are not sure about the authenticity of the email you received from PostmanSG, you can check your email header.
If you are using Outlook, you can go to File > Properties > Internet headers.
What you want to see is the part of the header that shows the authentication results. You should be able to find something like this (see below).
In Gmail, you need to go to show original to check the SPF, DKIM, and DMARC records.
Make sure that SPF, DKIM, and DMARC are all 'PASS'