I receive a 550 553 or relay prohibited error when sending email messages

Symptom

When you're away from home and send an email message using your home email account, your email message might be returned with a 550, 553, or relay-prohibited error message. The same situation could occur when you're away from your office and try to send an email message using your work email account.

Summary

Relaying occurs when an email message is sent to an email address whose domain (the name after the @ symbol, such as adatum.com) is not processed by the Simple Mail Transfer Protocol (SMTP) or outgoing server that the sender is requesting to deliver the message. The SMTP server must connect to another SMTP server to relay the message.

When you send an email message that encounters a relay error, your SMTP (outgoing) email server might return your email message with an error message such as one of the following:

• "The message could not be sent because one of the recipients was rejected by the server. The rejected email address was '<someone@example.com>'. Subject: '<Test>', Account: '<Test>', Server: '<smtp.example.com>', Protocol: SMTP, Server Response: '550 <someone@example.com>... Relaying Denied', Port: 25, Secure (SSL): No, Server Error: 550, Error Number: 0x800CCC79."

• "The message could not be sent because one of the recipients was rejected by the server. The rejected email address was '<email address>'. Subject '<Test>', Account: '<Test>', Server: '<smtp.example.com>', Protocol: SMTP, Server Response: '553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)', Port: 25, Secure(SSL): No, Server Error: 553, Error Number: 0x800CCC79."

The exact error message might vary, depending on your Internet service provider (ISP). Some ISPs might not return an error message when they detect outgoing messages as unsolicited commercial email. In these cases, your message might appear to be sent normally — it leaves your Outlook Outbox and appears in your Sent Items — but it's never actually delivered to the recipient.

Your message was rejected because the SMTP (outgoing) email server did not recognize you as an authorized user.

SMTP is the protocol (standards that computers use to communicate with each other) that most email servers use to send email messages across the Internet. When you use an email program, such as Outlook, that lets you store your email messages on your computer, you need access to an SMTP server to send email messages.

Note: Web email systems similar to Windows Live Mail and Yahoo! Mail are used differently, and this topic does not apply to those email accounts.

Junk email and open relays

Unsolicited commercial email is sometimes called junk mail or spam. The main reason that junk email continues to increase in volume is that it costs the person who sends it virtually nothing to send; in fact, the senders don't even have to send the junk email through the SMTP (outgoing) email server of their own ISP.

The basic structure of the Internet was designed before anyone considered the implications of providing the ability to send millions of pieces of junk email for little cost. Spammers use the relaying ability of SMTP servers to mask the true origin of junk email by relaying it through third-party servers that permit such open relays. This makes the junk email appear to come from the site that relays the message and conceals the identity of the real sender.

Until recently, most SMTP email servers worked on an open trust system. Under this system, anyone, anywhere could submit an email message to an SMTP server, and the server would accept it and forward it to a recipient or to another email server where the recipient's mailbox was located. Under a so-called open relay server, there were no restrictions on who was allowed to send via the SMTP server.

ISP restrictions on relay email messages

As junk email volumes increased, network administrators — the people responsible for managing your ISP servers — began placing restrictions on their SMTP email servers. These restrictions help prevent just anyone from using or abusing an email server. Think of it like this: a telephone in the lobby of your organization used to be available for anyone to use whether they worked at your organization or not. Now, only the employees are permitted to use that phone.

There are several types of restrictions in use today:

• Require SMTP authentication     Just as you must use a password to access your POP3 (incoming) server for your email messages, this option requires that you provide a user name and password to send email messages through the SMTP server. Usually, these are the same user name and password used for the POP3 server; but they can be unique.

• Require that you connect to the ISP POP3 (incoming) email server first     When you connect to retrieve your new email messages, you typically connect to a POP3 (incoming) email server. You are required to provide a user name and password to access your mailbox. A network administrator can configure the server so that if you first connect and authenticate with the POP3 email server, it will approve any request that you make to send an email message through the normally restricted SMTP outbound server.

• Require that you connect from an authorized network location     When you're at home and you dial your ISP or if you have a cable or DSL modem, you are directly connecting to the ISP network. You are trusted in that you have an account with the ISP with a user name and password. You are authorized to use the SMTP server to send email messages because you are a customer.

• Require that you connect from a specific IP address or range of IP addresses Your ISP might authorize access to the SMTP server to people who are not connected directly to the network. A remote user at an office can use this option. However, a major problem is that many places have what are called dynamic IP addresses. Each time that you connect, you're not assured of having the same IP address. Some companies might have a reserved block or range of IP addresses. Your ISP can authorize connections from those IP addresses as approved users. Your ISP can provide additional information.

There are many possible scenarios for relaying. The following are the most common situations. See if one matches your situation.

Scenario	Is this relaying?
You're at home and have an ISP account that ends with @proseware.com that you dial or connect to with a cable or DSL modem. You send an email message to another person whose email address also ends with @proseware.com.	No. Your mail should be processed normally.
Same as the first scenario, except that you send an email message to a person whose email address ends with @adatum.com.	Yes, but it is not blocked. You are directly connected to your ISP and thus are authorized to send mail through the ISP's SMTP (outgoing) server to any email address, regardless of where the recipient's mailbox is.
You're at work. Your work email address ends with @thephone-company.com, and you have a home ISP account that ends with @proseware.com that you dial or connect to with a cable or DSL modem. In Outlook, you have the same SMTP server settings that you use at home. You send an email message to a person whose email address also ends with @proseware.com.	No. Your mail should be processed normally.
Same as the preceding scenario, except that you send an email message to a person whose email address ends in @adatum.com.	Yes, and this message could be blocked as relay mail. You're attempting to use your home ISP's SMTP (outgoing) server while not connected to the ISP network. The SMTP server can't validate you as an authorized subscriber of the ISP. In addition, you're asking that SMTP server to take your message and then connect to another SMTP server for delivery to the recipient's mailbox.
You're staying at a hotel or using an airport Internet kiosk that provides Internet access. You have a home ISP account that ends with @proseware.com that you dial or connect to with a cable or DSL modem. In Outlook, you have the same SMTP server settings that you use at home. You send an email message to another person whose email address also ends with @proseware.com.	No. Your mail should be processed normally.
Same as the preceding scenario, except that you send an email message to a person whose email address ends in @adatum.com.	Yes, and this message could be blocked as relay mail. You're attempting to use your home ISP's SMTP (outgoing) server while not connected to the ISP network. The SMTP server can't validate you as an authorized subscriber of the ISP. In addition, you're asking the SMTP server to take your message and then connect to another SMTP server for delivery to the recipient's mailbox.

Resolutions

If you're using a scenario that is considered relay, you must send the message through your current connection's server. This means if you're at work or away from home and not using your ISP to connect to the Internet, and you want to send a message from your home ISP email account, you must change your email account settings to specify the SMTP server used at your location, for example, your work SMTP server. For step-by-step procedures, go to Change email account settings.

If this solution doesn't work for you, or you prefer to use your home ISP account, your next step is to contact your ISP and ask if any of the options described earlier are available to you. For the first two restrictions — require SMTP authentication or require that you connect to the ISP POP3 (incoming) mail server first — you can make the changes in Outlook in Account Settings. For procedures, go to Change email account settings.

Still can't send email messages?

You changed your SMTP settings in Outlook or found an option that should allow you to send your email message. However, you still can't send mail and you receive an error message.

It's possible you did everything right but encountered another safety feature that network administrators use to prevent identity spoofing. Identity spoofing is simply a way of sending an email message and disguising who you really are.

Outlook, like most email programs, allows you to specify the "display name" and the return email address that appear if someone clicks Reply to your message. Junk email nearly always contains false information in these fields. Do you really think that those messages you received about a get-rich-quick scheme came from a supermodel or a world leader?

To prevent identity spoofing, some ISPs restrict the insertion of false information in the reply email address field. For example, if your ISP domain name ends in proseware.com, the ISP might not permit you to set your email return address as terri@contoso.com. This restriction is not as commonly used as the restrictions described earlier, but it can be applied to all users, regardless of their location and connection. There's no alternative. If your server administrator is using this method, you must specify a return email address domain that matches your current connection.