Understand outbound SMTP problems in Azure VMs

Q. I have problems sending SMTP email from my Azure VM?

A. This is to be expected. Not because of any blocking done by Azure related to outbound TCP port 25 (used by SMTP) but because of the nature of the cloud which would apply to any cloud vendor.

Azure has a large range of public IP ranges that traffic is sent from. Spammers spin up services in a public cloud service (like Azure, AWS etc) and send spam. This results in that IP address being blocked by 3rd party block lists. The spammer takes down the service (or more likely Azure security team takes it down once its detected) but that IP address is now used by another customer but its still on the blocked list. Now consider this constantly happening and result is large blocks of ANY cloud provider IP range ending up on black lists for email. It is for this reason sending SMTP from cloud services can be highly inconsistent and the guidance is not to send email from Azure directly but rather use an email sending service for bulk email or hook into services such as Office 365 which have their own blocks of IP ranges separate from the general Azure services.