Don't forget SMTP when migrating to Exchange Online
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With the variety of options, no single SMTP option is a one size fits all solution. Per Microsoft, SMTP Submission is the recommended option for simplicity and security combined. However, some devices and applications may not support TLS. There may be some devices that are only intended to send e-mail to internal recipients, which would make Direct Send an ideal fit. Having an SMTP Relay server provides a central administration point for greater control over e-mail relay. SMTP Relay can also be beneficial if a certain device or application has no direct internet access and can only communicate to the internal network. Thankfully, you are not restricted to using only one option. All options can be used in any combination to meet the needs of the entire organization. With proper planning, transitioning all devices and applications over to Office 365 SMTP services can be a smooth and painless process.