Email Error 551 is an SMTP error commonly seen in MailEnable servers, and is seen in two forms:
551 Sender address is not valid for your login
551 This mail server requires authentication before sending mail from a locally hosted domain
MailEnable is widely used in Plesk web hosting servers, and these errors are often reported to web hosting providers. As part of our Outsourced Web Hosting Support services, we use multiple strategies to resolve Error 551.
But before we get into the details, let’s see what this error means:
What is SMTP Email Error 551?
In SMTP protocol, Error 551 is shown when a sender is not found to be authorized to relay mail through the server. This is a strategy to combat spam. It can be implemented in two forms:
1. Sender or Recipient mail address not authorized
For an SMTP server to accept a mail into the queue, either the Sender’s Mail Address or the Recipient’s Mail Address should be hosted in the SMTP server.
In MailEnable servers, this is controlled by a setting called “Authenticated senders must use valid sender address“.
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2. Sender not authenticated
A mail is accepted by the server only if the sender is authenticated using a password.
In MailEnable servers, the setting “Senders from local domains must authenticate to send email” is used to enable this feature.
How to fix Error 551 in MailEnable servers
Mail sending through SMTP authentication is a powerful way to block spammers. So, we recommend web hosting providers to keep the setting “Senders from local domains must authenticate to send email” enabled at all times.
However, the setting “Authenticated senders must use valid sender address” is seen to cause more harm than good. So, for servers in which Error 551 is common, we recommend disabling this option.
It can be done by going to SMTP Properties >> Security tab.
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