Sending a group e-mail and the gmail recipients are bouncing back to me

When group messages to Gmail recipients are bouncing, the cause is typically one of the following, based on the error text in the non‑delivery report (NDR):

1. Gmail is blocking messages as spam or low‑reputation domain
   If the NDR from Gmail looks like:

   550 5.7.1 … Gmail has detected that this message is likely suspicious due to the very low reputation of the sending domain …

   Then Gmail is rejecting the mail because of the sending domain’s reputation. The documented workaround (now fixed for Outlook.com country domains) was:
   • Use an Outlook.com alias as the From address when sending to Gmail. See: Add or remove an email alias in Outlook.com in the referenced article.
   For Microsoft 365 group mailings, check:
   • That the domain’s DNS (SPF, DKIM, DMARC) is correctly configured.
   • That bulk/group messages are not being sent in a way that looks like spam (sudden large volume, spam‑like content, etc.).
2. Recipient group restrictions in Exchange Online (codes 550 5.7.x)
   If the Gmail addresses are part of a distribution group or Microsoft 365 group and the NDR shows one of these codes, use the corresponding fix:
   • 550 5.7.1 to a group:
     • Add the sender to the group’s allowed senders list (create a mail contact or mail user if external).
     • Or configure the group to accept messages from external senders.
     • Or adjust any mail flow rule that is blocking the sender or sender domain.
   • 550 5.7.124 / 5.7.124:
     • The sender is not in the group’s allowed senders list.
     • Either:
       • Have the sender join the group (if it’s in the same organization and allows join requests), then send again, or
       • As group owner/admin, add the sender (or a group they belong to) to the allowed senders list.
   • 550 5.7.133:
     • The group is configured to reject messages from external senders.
     • As group owner/admin, change the group settings to allow external senders if that’s desired.
3. What the sender can do vs. what the admin must do
   • If acting as the sender:
     • Open the NDR and note the exact error code (e.g., 550 5.7.1, 5.7.124, 5.7.133, or Gmail spam/low reputation text).
     • If the NDR indicates group restrictions (5.7.1, 5.7.124, 5.7.133), contact the group owner or Microsoft 365 email admin and provide the NDR so they can change the group settings.
   • If acting as the group owner or Microsoft 365 admin:
     • For 5.7.1/5.7.124: edit the distribution group/Microsoft 365 group in Exchange Online and:
       • Add the sender to allowed senders, or
       • Relax restrictions so group members or external senders can post as required.
     • For 5.7.133: enable “accept messages from external senders” for that group if external Gmail recipients should be allowed.
4. If only Gmail recipients bounce, others succeed
   • Check the NDR text for Gmail‑specific spam/low‑reputation wording as above.
   • Ensure that the From domain is not an Outlook.com country domain that recently had reputation issues; if it is, use an Outlook.com alias as From (per the documented workaround) until reputation issues are resolved.

In summary, inspect the NDR for the exact error code and text. If it’s an Exchange Online 5.7.x group restriction, adjust group allowed‑sender or external‑sender settings. If it’s a Gmail spam/low‑reputation rejection, improve domain reputation and, where applicable, use an Outlook.com alias as From when sending to Gmail.

References:

• Fixes or workarounds for recent issues on Outlook.com
• Fix NDR error "550 5.7.1" in Exchange Online
• Fix NDR error "550 5.7.124" or "5.7.124" in Exchange Online
• Fix NDR error "550 5.7.133" in Exchange Online