Solve error “unable to relay for email address” in 3 steps

Emails are convenient, but email errors can be tiresome.

At Bobcares, we often resolve such email errors as a part of our Technical Support Services for web hosting companies.

Today, we’ll see what causes “unable to relay for email address” errors during email delivery and how we fix that error in 3 steps.

What causes the error “unable to relay for email address”

When sending mails from a contact form or mail client such as Outlook or command line in Windows Exchange servers, users sometimes get this error.

The following recipient(s) could not be reached:

550 5.7.1 Unable to relay for email address


This error means that the sender is not allowed to relay through the server. Most mail servers have security measures to prevent them from being open relays.

An open relay is an insecure mail server that allows anybody to connect to it and send mails without proper authentication. Open relays can cause spamming and resource abuse.

When users attempt to send mails using a mail server that is not an open relay, it will reject that mail delivery with the error “unable to relay for email address”.

In Exchange servers, there are restrictions in the STMP server that prevents it from being used as an open relay. The main reasons for this error are:

1. SMTP authentication

When users send mails via email clients such as Outlook or via contact mail forms, they have to provide their login credentials to authenticate.

Mail server checks the username and password provided, and validates that connection. If these details are wrong or missing, mail server will reject sending mails through it.

2. Port blocks

By default, the SMTP port is 25. But in majority servers, this port would be changed into some alternate port such as 587, to reduce spamming.

If the mail client of the user is configured to connect to port 25, but the port is blocked, user would get email rejections with the error ‘unable to relay for email address’.

3. IP restrictions

Receive connectors in mail servers are used to configure how the mail server listens to SMTP connections. By default, the receive connector in Exchange server only allows to relay messages to internal users.

If users try to relay mails via a mail server, the IP address for that user should be allowed in the receive connectors. Otherwise it will throw up error ‘unable to relay for email address’.

How to fix the error “unable to relay for email address”

1. Mail client configuration

The first thing to check is whether the user’s mail client is configured correctly. This include settings like:

  • Email account username
  • Email account password
  • Incoming and Outgoing server details
  • SMTP port
  • SSL support

For the mails to get delivered fine, the option ‘My Outgoing Server SMTP Requires authentication’ has to be enabled in the Outlook settings.

This will ensure that users validate their accounts and send mails after authenticating to the mail server.

2. Configure Receive connectors

For relaying mails through a mail server, the user’s IP address should be added to allowed list of IP addresses in the Receive connectors.

When valid customers want to relay through the mail server, we allow their IP address to the SMTP server’s list of allowed servers. But caution has to be exercised to prevent abuse or spamming from that IP.

Adequate permissions are also set for the newly added Receive connectors. It is not at all recommended to allow too many or All IP addresses to relay through the server.

3. SMTP ports

Ruling out blocks on SMTP ports is a major aspect. Connectivity issues, antivirus, firewall rules, etc. can lead to port blocks and mail delivery errors.

We allow appropriate rules for allowing SMTP connectivity for valid users. For further email security, we configure SPF records, RDNS, DKIM rules, etc. for domains.


Emails are the major mode of communication for online businesses. Email errors can be vexing for users. Today, we saw how our Outsourced Tech Support team easily fix the error ‘unable to relay for email address’ for our customers.



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