553, Sorry the domain isn’t in my list of allowed rcpthosts.

You must use SMTP authentication to send email through our servers. In Outlook Express it is found in Tools, Accounts. Choose the Properties for the affected mail account and click on servers. You will see a checkbox for "My server requires authentication". Please check that box and save the settings.

====================================================================
The error generally occures sending emails before receiving. i.e. start the POP3 service before SMTP.
1. You cannot send an email before receiving.
2. Some internet services block third party email servers from functioning. Their reason is that they want to prevent you from using these servers to send SPAM (junk mail). In this cases you have two options. You can either contact the internet service provider (ISP) and have them unblock your account, or you can use your ISP's SMTP server instead of your domains'. When you send email, it still appears as if it is coming from your domain and it makes no difference.

====================================================================
The issue starts when email clients (such as Outlook) have an option set to send messages immediately. When you send a message immediately, your email program skips the authentication (verification) process and tries to send without receiving. If this happens after the 20 minute gateway closes, the server gets stuck because it has no idea which user is sending this email. It then stops, and says (not literally), "I do not know you; therefore I will not let your mail through". Basically, "553, sorry, that domain isn't in my list of allowed rcpthosts" means that the server is not recognizing you. So what are some ways to avoid this? We have two easy methods (A and B) to stop this from happening in Outlook and Outlook Express.

A. Click Send/Receive before composing an email. This way, you have 20 minutes to send the email and will not see the error.
B. (Recommended) you can tell the email client NOT to send messages immediately. This is the recommended method. This way, you compose a message; take as long as you need, click send, and when you're ready to send it out, click Send/Receive.

MS OUTLOOK:
-Open Outlook - Click on Tools and choose Options
- Click the Mail Setup Tab - Uncheck "Send Immediately When Connection"
- Click OK and make sure the Outbox is empty.
- Close Outlook and open it once more.
- From this point on, you should no longer see this error. MS OUTLOOK EXPRESS:
- Open Outlook Express
- Click on Tools and choose Options
- Click the SEND Tab
- Uncheck "Send Messages Immediately"
- Click OK and make sure the Outbox is empty.
- Close Outlook Express and open it once more.
- From this point on, you should no longer see this error.

If you are using a program other than Outlook and see this error, please contact the manufacturer for ways to configure their email client to do the same in order to avoid the error.

====================================================================
The error generally occures sending emails before receiving.
i.e. start the POP3 service before SMTP.
1. You cannot send an email before receiving.
2. Some internet services block third party email servers from functioning. Their reason is that they want to prevent you from using these servers to send SPAM (junk mail). In this cases you have two options. You can either contact the internet service provider (ISP) and have them unblock your account, or you can use your ISP's SMTP server instead of your domains'. When you send email, it still appears as if it is coming from your domain and it makes no difference.