Email to Outlook, Hotmail, or T-Online delayed or rejected

Does mail to an address at Outlook.com, Hotmail, Live.com, a company with Microsoft 365, or T-Online arrive hours late, or does it bounce back? Microsoft and T-Online use their own reputation system for sending servers, in addition to the usual checks. Mail that arrives at Gmail without any problems can be delayed or refused there. In this article you'll read what their notifications mean, what you can do, and what we do.

Microsoft: temporarily throttled (4.7.500)

Did you get a deferral message with this notification?

451 4.7.500 Server busy. Please try again later from [203.0.113.25]. (AS843)

In that case, Microsoft's server isn't actually busy. Microsoft temporarily throttles the IP address of the sending server to assess the traffic, for example if mail volume from it suddenly increases. Microsoft uses the same range of codes under the name Access denied, please try again later, and lifts the restriction automatically once the mail turns out to be fine.

• This is a temporary error (a code starting with 4). Our server keeps the mail and retries automatically. Usually the mail still arrives after one or more attempts, sometimes only after a few hours.
• Don't resend the mail. That would make it arrive twice, and more mail is likely to make the throttling worse.
• Does a mail really need to arrive right away, or does this keep happening? Then send us the deferral message.

Microsoft: IP address blocked (5.7.606 and 5.7.708)

550 5.7.606 Access denied, banned sending IP [203.0.113.25]. To request removal from this list please visit https://sender.office.com/ and follow the directions.

This is a permanent rejection: Microsoft has blocked the IP address of the sending server. Requesting removal via Microsoft's delisting portal is only useful for the administrator of that IP address, once the cause has been resolved. If your mail goes through a WebOké server, we'll take care of that. Send us the full bounce so we know which server is involved. Microsoft doesn't have a manual whitelist: it remains an automated system.

Do you see 550 5.7.708 Access denied, traffic not accepted from this IP? Then you're sending via Microsoft 365, and Microsoft is no longer accepting traffic from that environment, usually following abuse. You resolve this through Microsoft support, via the administrator of your Microsoft 365.

Microsoft: in the Junk Email folder

Often mail isn't rejected, but ends up in the Junk Email folder, especially from senders Microsoft doesn't recognise yet. In that case there's no bounce. What helps:

• Make sure SPF, DKIM and DMARC are set up correctly. Check your domain with the E-mail check and read What is DMARC? A plain-language explanation.
• Ask regular contacts to add your address to their contacts or safe senders, and to mark mail that still ends up in Junk Email as Not junk. How to do this in Outlook is explained by Microsoft.
• Don't send newsletters from your regular mailbox, but use a newsletter service instead.

Do you send a lot of mail to Outlook.com and get 550 5.7.515? Then Microsoft requires that both SPF and DKIM pass and that you have a DMARC record. See Microsoft's explanation.

T-Online: its own rules

T-Online is the largest mail provider in Germany. The T-Online postmaster page lists the requirements for sending servers. The most important ones:

• The IP address of the sending server has a correct reverse DNS (PTR): the IP address points to a name, and that name points back to the same IP address.
• The domain in that name has a website stating how to reach the server's administrator. In Germany, such a page is called the Impressum.
• No mail directly from residential connections or anonymous servers, and not too many simultaneous connections.

A rejected mail might look like this, for example:

554 IP=203.0.113.25 - None/bad reputation. Ask your postmaster for help or to contact tobr@rx.t-online.de for reset. (NOWL)

This means: this IP address currently can't deliver to T-Online. This concerns the server, not your domain or your mail. T-Online doesn't offer whitelisting on request from a customer. Only the administrator of the server can request a release.

What you do:

1. Send us the full bounce via a ticket in the Support Centre. Don't email T-Online yourself: only we can request a release for our servers.
2. We request a release from T-Online for our servers. This can take a while, and T-Online doesn't always send a confirmation.
3. Is a mail to someone at T-Online urgent? Then temporarily agree on a different address or another way to get in touch.

Do you see 421 Maximum parallel connections for your IP-Address reached? Then it's temporary. Our server will try again later and the mail usually arrives on its own.

Check it yourself first

With E-mail Tracing in DirectAdmin you can see whether a mail was delivered, deferred, or failed, along with the recipient server's response. See Checking whether an email has arrived with E-mail Tracing. What other error codes mean is explained in Mail delivery failed: how to read the error message and what the codes mean. When sending us a ticket, please include the full bounce, along with the recipient's address and the time.

Also see in the DirectAdmin manual: My mail ends up in the recipient's spam and Sending refused: blacklists and limits.