SMTP code 5.7.703

What the knowledge base's own tables say about this code, and which articles say it — a description of the reply, never a placement prediction.

This line comes from a knowledge base article on Microsoft, not on the standard.

Blocked entity
Domain or email address
Effect
The message is treated as high confidence phishing and quarantined (not merely treated as spam). The tenant's own users also cannot send to it (550 5.7.703 ... blocked by your organization using Tenant Allow Block List)
From Microsoft Filtering Internals (EOP / Defender for Office 365) · Tenant Allow/Block List (TABL): what recipient tenants control