Sending, receiving, and organizing email in Outlook on the web for business
Hi,
As the forum’s AI recommended, checking the recipient count is a useful first step. For mailboxes hosted in Exchange Online, Microsoft documents a limit of 10 individual recipients in a forward or redirect Inbox rule. This also applies to internal recipients.
Could you confirm how many addresses are in the “Forward to” action when it stops working, and whether these are individual addresses or one distribution group address? The successful smaller test makes this worth checking, but it does not establish the cause on its own.
If you have more than 10 individual recipients, ask your Microsoft 365 administrator to create a distribution group containing the intended recipients, then use the group’s email address in the rule instead of listing each person separately. Keep the existing “From” condition and test with a new message from the specified sender.
Microsoft documents this workaround here: Diagnose Outlook Inbox rules that aren’t working.
If there are 10 or fewer recipients, or you already use a distribution group, please confirm whether nobody receives the forwarded message or only some recipients are affected, and whether any delivery failure message appears.
Also review the rules above this one under Settings > Mail > Rules. An earlier matching rule with “Stop processing more rules” enabled can prevent this forwarding rule from running. See Stop processing more rules in Outlook.
Please hide email addresses and other personal information if you share screenshots or error messages.