One possible reason may be SMTP Authentication. SMTP Authentication checks to see that you are authorized to send email before allowing you to relay messages through the server. This means you MUST configure your email client to authenticate with your username and password.
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You may refer to the below Tutorials on how to configure your email client.
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We use the standard email configuration for all of our services. Below are the configuration that commonly used by us :
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1. POP configurationÂ
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  NON-SSL
  Incoming mail server POP:   mail.domain.com
  Outgoing mail server SMTP : mail.domain.com
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  (replace your OWN domain name for domain.com)
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  SSL
  Incoming mail server POP:   serverhostname
  Outgoing mail server SMTP : serverhostname
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  (serverhostname refers to the name of the server that is hosting your email
  serverhostname can be found from your web hosting control panel server information section )
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NON-SSL
Incoming POP : 110
Outgoing for POP Â : 587
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SSL
Incoming POP : 995
Outgoing for POP : 465
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NOTE : If you choose SSL settings, check This server requires an encrypted connection (SSL) box.
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2. IMAP configurationÂ
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 NON-SSL
  Incoming mail server IMAP :  mail.domain.com
  Outgoing mail server SMTP : mail.domain.com
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  (replace your OWN domain name for domain)
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  SSL
  Incoming mail server IMAP :  serverhostname.com
  Outgoing mail server SMTP : serverhostname.com
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   (serverhostname refers to the name of the server that is hosting your email
  serverhostname can be found from your web hosting control panel server information section )
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NON-SSL
Incoming IMAP : 143
Outgoing for IMAP : 587
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SSL
Incoming IMAP : 993
Outgoing for IMAP : 465
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NOTE : If you choose SSL settings, check This server requires an encrypted connection (SSL) box.
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The reason we use this system is because if the server did not check before allowing you to send email, then anybody on the Internet could send email through the server, thus leaving it open to bulk mailers (Spammers).
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NOTES:
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- Some ISP's force you to use their SMTP servers to send mail by blocking your ability to make connections to external SMTP servers. Please check with your provider to see if they employ "port 25 blocking".
- If you are connect to a local net work with Firewall, kindly check with the Network Administrator to ensure that the Firewall does not employ "port 25 blocking".