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I2028: Error 550 when connecting to mail servers
Product: FreeProxy
Versions: FreeProxy, all versions
Note: 2028
Date: reported 21 November 2003

Issue Detail
I can receive email but can't send email and get an error 550.

When does it occur ?

Error 550 is sent to the email client by the SMTP server if it suspects you are attempting to "relay" email messages. The text associated with the 550 error varies.

This error does not normally occur when receiving email.

Typical connection example
Under normal circumstances, you connect to your ISP's email server and send and receive email. The ISP will allocate you one of their email addresses (eg: gregr@OmegaISP.com).

For example, the ISP is "OmegaISP.com". The user id is "gregr" and the connection is made using a dial-up modem.

The process would be as follows:

  • I dial the ISP (OmegaISP.com) using a modem and connect.
  • The ISP accepts my modem connection and allocates the connection one of their IP addresses
  • FreeProxy connects to smtp.OmegaISP.com and pop.OmegaISP.com to send and receive email from a connected client.
    How can it go wrong ?
Any one of the following can result in a 550 error, however some ISPs implement none of these measures whereas others implement some or all of them.
Restriction 1. The sender's email address must be the same domain as the ISP - internal connections
In your email client (like Outlook, Netscape email Outlook Express) your email address must have the same domain name as the ISP, or have one of the email addresses allocated to you by the ISP. So, if your email address is gregr@OmegaISP.com, and you are connected as described in the typical connection example above, your ISP may only allow the connection if your email address contains "@OmegaISP.com". It may not allow the action if your email address contains anything else. From the mail server's point of view, they would be receiving a request to send some other domain's email; which is also called mail relay. Many ISPs forbid mail relaying.

Restriction 2. The recipient email address must be the same domain as the ISP - external connections
When an email arrives at your ISP's smtp server from an external mail source, it must have the ISP's domain name in the recipient address field.
So, if you send me an email from your email address, say from Ludwig@beethoven.de, then when the email arrives at mail.OmegaISP.com (my smtp server), then it MUST contain "@OmegaISP.com" in the domain name. Sending the email to "gregr@OmegaISP.com" would then reach me without a problem. If the domain name is incorrect, then the OmegaISP may suspect you are attempting to relay email and reject the email with a 550 error. This is called a relay, that is you are requesting a mail server to accept an email address that needs to be relayed to some other smtp server.

Restriction 3. You must have an acceptable IP address
Refer back to the Typical Connection Example. You will notice that the ISP allocated your connection one of its range of IP addresses.

In a slightly different scenario, instead of connecting to OmegaISP.com using a dial-up connection, you connect via your cable connection to a different ISP, say BigRiver.com. BigRiver then allocates you one of their IP addresses. Because the config in the email client and FreeProxy are the same, options 1 and 2 above are satisfied however the smtp server can make one more check; your IP address must also be one of the ones allocated by the ISP.

The message would be rejected in this scenario:
    You are connected to an ISP other than OmegaISP.com (eg via cable connection to BigRiver.com)
  • Your email recipient address is (for example) gregr@OmegaISP.com and your recipient address is for example "antoinio@vivaldi.it" (an "External" account)
  • You connect to mail.OmegaISP.com and attempt to deliver the email to "antonio".

Because mail.OmegaISP.com is connected to from BigRiver.com, the 550 error is returned. It would not have been returned if the recipient was @OmegaISP.com. In this case the rejection is based on IP address.

These limitations are not contained in FreeProxy but rather the security procedures of your ISP. These restrictions will apply whether you connect directly or via FreeProxy. You should be able to define as many POP accounts as you need assuming restriction 3 (above) does not apply to the POP server. You may then have to only send email via the connected ISP.
Alternately, some ISPs allow you to relay email a long as you request the service and you let them know how you will be connecting to their email server. So, using the examples above, if you notify them that you will be connecting to mail.OmegaISP.com from BigRiver.com, they may allow you to relay your emails as long as your from address is the one allocated to you. Discuss this with your ISP.

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