How to troubleshooting the mail error: 550 unroutable address in linux.

Email is abbreviated by Electronic Mail. Information stored on a computer
that is exchanged between two users over telecommunications. E-mail is a
message that may contain text, files, images, or other attachments sent
through a network to a specified individual or group of individuals. When
sending an email it should required some fields of data.Three are To ,
From if you are replying to a message, the To: and From: fields are
automatically filled out. The Subject should consist of a few words
describing the e-mail's contents.Finally, the Message Body is the location
you type your main message.

The error ‘550-Unroutable address’ occurs due to the routing issues that
happen in this mail delivery process, which can be due to either the
recipient or sender problems.

The unroutable mail domain error message is a very general error message
of mail server which could means a lot of things and possible causes of
error. This type of error can be due to a variety of reasons. The
recipient's email address might be misspelled. You also might have exceeded
the maximum number of e-mails you are allowed to send per hour, in which
case you should allow some time and then resend the mail. Your mail server
might also be unable to determine the correct route to send your email
message because of a DNS configuration error with the recipient domain's MX
records. This can be addressed by the owner of the recipient address.

Reasons and solutions:

1.Sending Emails to a Non-Existent Recipient Domain :

When the email is delivered to a non-existing recipient domain, it shows a
‘550-Unroutable address’ error. Another reason for the error is when the
user sends an email with a wrong typo in the email address.So if the
recipient email addresses are wrong, the emails would not be delivered and
would bounce back to the sender by giving an error message.

Solution : The solution here is to validate the recipient email address
while sending the emails and check whether the domain is neither expired
nor deactivated

2.Incorrect MX Records for the Recipient Domain :

The sender email server will not be able to route correctly to the
recipient email server unless the MX records are set properly for the
recipient domain. If there is any error in the DNS settings for the
recipient domain, the other external emails servers render the email
delivery as unreachable.

Solution: So always verify whether the MX records are correctly set for the
recipient or not.

3.Email exceeds per hour :

In email server such as Exim, the number of emails a domain can send out
every hour has a limit. After the limit exceeds, the emails would bounce
back to the sender by displaying the error ‘550-Unroutable address’.Wrong
routing of the domain, where the mails are routing to remote server instead
of local mail server, can also end up giving email error 550.

Anti-spam firewalls or certain security rules in the recipient mail server
can lead to email bounces with 550 error.

4.Verification Failure:

The sender’s mail server or recipient’s mail server may also has sender
verification turns on, in which if the verification fails, you will have
550 unroutable error.

However, the root cause is still related to DNS problem mentioned above as
the setting enables check and verify if the MX record of the sender is
valid.