Hi @Meher, Shankar (CORP) , Welcome to Microsoft Q&A,
Regarding how to use c# to connect to Exchange Online and send emails, you need to check this document specifically: Enable or disable authenticated client SMTP submission (SMTP AUTH) in Exchange Online (These settings only apply to mailboxes that are hosted in Exchange Online (Office 365 or Microsoft 365).) This forum does not support other email addresses.
You need to Enable SMTP AUTH for specific mailboxes.
You can also check out this reference list.
Best Regards,
Jiale
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