What is SMTP?


SMTP (Simple Mail Transfer Protocol) is TCP/IP Protocol used for sending and receiving e-mail. However, since it is limited in its ability to queue messages at the receiving end, it is usually used with one of two other protocols, POP3 or IMAP, that let the user save messages in a server mailbox and download them periodically from the server. In other words, users typically use a program that uses SMTP for sending e-mail and either POP3 or IMAP for receiving e-mail.


SMTP is being implemented to operate over the Internet port 25. Since it has been limited in its ability for queue messages at the receiving end, it is basically used with one of two other protocols such as POP3 or IMAP, it allows the user to save messages in a server mailbox and download them regularly from the server. Mostly. users use a program which uses SMTP for sending the e-mail and POP3 or IMAP for receiving e-mail. On Linux hosting plan, send mail is the commonly used SMTP server for e-mail. The Sendmail consists of a POP3 server.


SMTP and mail services on Windows Platform specific to HostingRaja solutions

We use one of the most popular Email solution on Windows hosting platform, Which is called us Mailenable. MailEnable Standard Edition comes  SMTP, IMAP, POP3 and Webmail services for Microsoft Windows servers. It comes with wonderful webmail service, Which can be accessed from Android, or any mobile screen or Desktop for office purpose.

How to resolve SMTP issues?

Incorrect username or password

You may get a popup saying "Enter network password" if you use Outlook.

  • Use your full email address for your username - it should look like This email address is being protected from spambots. You need JavaScript enabled to view it..
  • Test your username and password in webmail at http://webmail.example.com. Replace example.com with your own domain.
  • Reset your email password for assistance.
  • In Apple Mail especially, try re-typing your username and password, even if they are correct. Sometimes the stored password keychain gets corrupted.

Mailbox full / Quota Exceeded / Could not create output file

  • This error indicates that the recipient's mailbox is full. You should receive the same error if you try to send to that particular recipient from a various email address. Please let them also know about this problem.
  • It is also possible to receive a disk space error if you store a copy of sent messages on the server for that mailbox, and the mailbox may nearing its capacity.


Rejected: User does not exist / No such address

This error indicates that recipient address may be misspelled, or may recently have been deleted as a mailbox on the recipient side. If your recipient has recently changed their DNS or their mail provider, you may need to wait 24-48 hours for their address to be switched to  new server.


Incorrect SMTP Configuration Files

Make sure your SMTP configuration files are present or not and configured correctly.


You can also rebuild your mail configuration files with the following command:

/usr/local/psa/admin/sbin/mchk --with-spam


Run the following command in SSH:

ls -l /etc/xinetd.d/smtp*


You should see the following results:

-rw-r--r-- 1 root root 416 Sep  2 15:20 /etc/xinetd.d/smtp_psa

-rw-r--r-- 1 root root 417 Sep  2 15:20 /etc/xinetd.d/smtps_psa


The names should have underscores, and the permissions and ownership should match those shown above.


If underscores are missing, you can restart the mail service with the blow command:


/usr/local/psa/admin/bin/mailmng -o ; /usr/local/psa/admin/bin/mailmng -p ; /etc/init.d/xinetd restart ; /etc/init.d/qmail restart

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