cannot send emails office 365 using several bcc contacts

Office 365 is likely rejecting the messages because they look like bulk/anonymous mail and/or are hitting anti-spam limits when many recipients are placed in Bcc.

Use one of these approaches instead of a single message with 150 Bcc recipients:

1. Use a proper distribution method
   • Create a distribution list or contact group for the golf members and send to that group instead of 150 individual Bcc addresses. This avoids an oversized Bcc list and is less likely to be treated as spam.
2. Reduce recipients per message
   • If a list or group is not possible, split the send into multiple messages with fewer recipients in Bcc (for example, batches of 25–50) to stay under throttling and anti-spam thresholds.
3. Check for error 5.7.1 or permission issues
   • If the non-delivery report shows error 5.7.1, the rejection is due to permission or routing/relay restrictions between the sending server and recipients. In that case, send directly from the Office 365 mailbox (not via an intermediate server) and avoid relaying through servers that are not allowed to send for the domain.
4. Ensure the sending method is supported
   • If messages are being sent via an application, printer, or LOB tool using direct send or relay, verify that it is configured according to Microsoft 365 guidance and that the IP address is not on a spam blocklist. If the IP is blocked, work with the blocklist owner to remove it and consider using authenticated SMTP or another supported method.

If large-group communication is a regular need, using a distribution group or a dedicated mailing solution is more reliable than repeated large Bcc sends.

References:

• Office 365 email delivery error 5.7.1
• How to set up a multifunction device or application to send email using Microsoft 365 or Office 365
• Fix issues with printers, scanners, and LOB apps that send email using Microsoft 365
• Outlook: BCC field is not displayed on some items in the Sent Items folder