Recipient's email rejecting some emails

@Lizbeth Loreto said

I have several contacts notifying me that some of their emails are rejected, receiving the following: Your message wasn't delivered because the recipient's email provider rejected it ... before they were able to send me emails and now are rejected.

The problem is that there are a lot of different reason why emails to you were sent back as undeliverable. We need to know the specific reason and that information is in the Non-Delivery Reports (NDR) that your senders receive when they try to send mail to you.

This may be difficult for you to do, but we really need to see one of the NDRs or at least the numeric code contained in the NDR that indicates why the email to you could not be delivered. They are usually something like this:

• 5.1.1 Bad destination mailbox address
• 5.1.10 Recipient not found

Can you contact one of your senders who is having this problem sending you emails and ask them send you a copy of the NDR thru another channel, or at least look at the NDR they got and see if there is one of the "Five Codes" anywhere in it?

The senders’ messages are being rejected by their email systems or by filtering between their systems and Outlook.com, not by the local Outlook.com blocked senders or contacts list. Having them in Contacts and not in the blocked list only affects local junk filtering; it does not override server-side blocks, reputation issues, or external blocklists.

Use these steps to move forward:

1. Confirm they are not blocked in Outlook.com
   • Sign in to Outlook.com.
   • Go to Settings > Mail (or Options) > Blocked senders / Block and allow.
   • Verify none of the affected senders’ addresses or domains (for example, @windermere..., @johnlscott..., @seattlecu..., @firstam...) are listed. If any are present, remove them.
2. Ask the senders to check their bounce (NDR) details
   • The error “Your message wasn't delivered because the recipient's email provider rejected it” is a generic summary. The full non‑delivery report (NDR) will contain an SMTP error code such as 550 5.1.8, 5.7.5xx, or text mentioning “blocked”, “listed in”, or a blacklist.
   • Have each sender open the delivery failure message and look for:
     • The SMTP status code (for example, 550 5.1.8, 5.7.606, 5.7.703, etc.).
     • Any mention of “blocked”, “spam”, “blacklist”, or services like “spamcop”, “spamhaus”, “dynablock”, “blackhole”.
3. If the NDR mentions “blocked”, “listed in”, or blacklist services
   • This indicates the sender’s server or account is being treated as a spam source.
   • The sender (or their IT/email provider) must:
     • Review the exact error text and any referenced blacklist.
     • Contact their own email provider or IT admin to request delisting/unblocking and to fix any spam/reputation issues on their side.
   • This aligns with the guidance that when errors include “blocked” or “listed in” with references to sites like “spamcop”, “dynablock”, “blackhole”, or “spamhaus”, the recipient’s server believes the sending server/account is a spam source and the sender must work with their provider to be unblocked.
4. If the NDR is from Microsoft 365 / Exchange Online with codes like 550 5.1.8 or 5.7.606–649
   • 550 5.1.8 Access denied, bad outbound sender means the sender’s account has likely exceeded sending limits or is suspected of sending spam. Their Microsoft 365 admin must:
     • Check if the account is compromised and secure it.
     • Unblock the user on the Restricted entities page in the Microsoft 365 Defender portal.
   • 5.7.606–649 Access denied, banned sending IP means the sending IP is banned. The sender’s admin must:
     • Verify they follow email deliverability best practices and that their IP is not compromised.
     • Use the delist portal referenced in the NDR to request removal from the blocked senders list.
5. If the NDR is 5.7.703 (Tenant Allow/Block List)
   • This applies when a Microsoft 365 organization has explicitly blocked a domain or address using the Tenant Allow/Block List. If the affected senders are in such a scenario, their admin must remove the block entry; only the sending or receiving tenant admin can change that list.
6. If messages are silently not arriving (no NDR to the sender)
   • Check Outlook.com junk and rules:
     • Review Junk Email folder for misclassified messages.
     • Review any inbox rules that might move or delete messages.
   • If nothing is found and senders do not receive NDRs, they should contact Microsoft support through their own provider’s channels or, for Outlook.com senders, via the Outlook.com support options.
7. Use Outlook.com support if the problem persists
   • If senders confirm their systems are not blocked or blacklisted and NDRs point to Outlook.com, use the Outlook.com in-product Help option to open a support request so support can review the mailbox and any server-side blocks affecting it.

In summary, the rejection is controlled by server-side filtering and reputation checks. Each affected sender (or their IT/email provider) must review their NDRs and work with their own provider or admin to remove any blocks or blacklist entries. Local Outlook.com contact and blocked lists do not override those server-side decisions.

References:

• Sent email in Outlook.com comes back "delivery failed"
• Email non-delivery reports and SMTP errors in Exchange Online
• NDR error code "550 5.1.8 Access denied, bad outbound sender"
• NDR error codes 550 5.7.703, 550 5.7.705, 550 5.7.708, and 550 5.7.750
• Where are my email messages or calendar events in Mail and Calendar for Windows 10?
• Receiving email from blocked senders in Outlook.com