When many applications send mail, the failing identity is hard to locate
Sender attribution, authentication alignment, provider policy and MIME evidence must be separated before explaining an Outlook or Hotmail delivery failure.
Checked 2026-10-09
What the sources establish
Google recommends identifying every application, server and provider that sends for a domain.
SPF, DKIM, DMARC alignment and sender reputation influence acceptance and placement decisions.
For high-volume senders to Outlook.com consumer domains, Microsoft documents SPF and DKIM pass, DMARC presence and alignment requirements; the current rejection is 550 5.7.515.
Malformed MIME, unsupported attachments or encoding defects can cause message-specific failures, but they are not a general explanation for Hotmail non-delivery.
PunchOp proposed response
Build a map from application to relay, envelope sender, From domain, DKIM selector and outbound IP.
Correlate the exact SMTP response or NDR with message headers and provider evidence.
Test authentication alignment separately from MIME structure, attachment type and size.
Report the narrowest supported cause and make missing recipient-side evidence visible.