Hi Omar, 

Please refer this documentation: https://docs.microsoft.com/en-us/azure/virtual-network/troubleshoot-outbound-smtp-connectivity

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Starting on November 15, 2017, outbound email messages that are sent directly to external domains (such as outlook.com and gmail.com) from a virtual machine (VM) are made available only to certain subscription types in Microsoft Azure. Outbound SMTP connections that use TCP port 25 were blocked. (Port 25 is primarily used for unauthenticated email delivery.)

This change in behavior applies only to new subscriptions and new deployments since November 15, 2017.

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Regards, 

Msrini