Sorry about the issue you faced.
There are two possible reasons for the error "Reason 550 #5.1.0 Address Rejected cannot sent to firstname.lastname@example.org" which appeared when you try sending a mail from your POP account configured in Zoho Mail, they are as follows.
a) Your mail service provider may require authentication to send mail from your POP account. Please enable "Outgoing Server Requires Authentication" in your POP account configured in Zoho Mail.
To enable Outgoing Server Requires Authentication, Go to Settings --> POP Accounts --> click 'Edit' --> enable 'Outgoing Mail Server requires authentication --> Save.
b) If option a) does not work, then this error might be due to your ISP don't allow relaying. Please contact your ISP regarding this error and resolve it at once.
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