I can send mails using an office 365 account (email@example.com) with host as smtp.office365.com in my website. But if I set a different email (firstname.lastname@example.org) for 'From' address I get the error,
Mailbox unavailable. The server response was: 5.7.60 SMTP; Client does not have permissions to send as this sender
I have given 'Send as' and 'Send on behalf of' permission for email@example.com as follows but still not working.
Both firstname.lastname@example.org and email@example.com are 'Exchange Online (Plan 1)' licensed accounts. What am I doing wrong? Thanks