Most new users get stuck with issues saying ‘Email address of the recipient was not resolved’. Like in case you want to send out an email from D365 but you don’t have the same saved in the system as a Contact/Lead or any other record.
And it resulted in this the big red cross like the one below –
There’s a simple setting for this in Settings > Administration > System Settings, under Email tab.
You can simply flip the switch on ‘Allow messages with unresolved email recipients to be sent’ to Yes.
And, now simply enter the Email address in the Recipient (To) field and let the email go!
Hope this quick tip was helpful!
D365 Demystified 
Great tip!
Would that work for appointments as well?
Thank you
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Thanks. Honestly, I haven’t tried. But I think it should work since it’s only dealing with the email address field and nothing specific to Email that Appointment might not have. I’ll try that. 🙂
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