Email Servers and Ports

Incoming and Outgoing Mail Servers


  • Incoming Mail Server (POP3, IMAP, HTTP)

The incoming mail server is the server associated with you email address account. There can not be more then one incoming mail server for an email account. In order to access your incoming messages, you need an email client: a program that can retrieve email from an email account, allowing a user to read, forward, delete, and reply to email messages. Depending on your mail server, you can use a dedicated email client (like Outlook Express) or a web browser (like Internet Explorer, for accessing web based email accounts, like Hotmail). The mail is held in storage on the incoming mail server until you download it. Once you have downloaded your mail from the mail server it cannot be downloaded again. In order to download your Email, you must have the correct settings configured in your Email client program. Most incoming mail servers are using one of the following protocols: IMAP, POP3, HTTP.

  • Outgoing Mail Server (SMTP)

This is the server used only to send emails (to transport them from your email client program to the receiver). Most outgoing mail servers are using the SMTP protocol (Simple Mail Transfer Protocol) for sending emails. Depending on your network settings, the outgoing mail server can belong to your ISP or to the server where you setup your email account. As an alternative, you can use a subscription based SMTP server (like, which will allow you to send emails from any email account you already own. Due to anti-spam reasons, most of outgoing mail servers will not let you send emails if you are not logged on their network. An open-relay server will allow you to use it for sending emails, no matter if you belong to its network group or not, thus it is a heaven for spammers.

dots eMail Servers and Ports


  • Email Ports

For networks, a port means an endpoint to a logical connection. The port number identifies what type of port it is. Here are the default email ports for:

    POP3 – port 110
    IMAP – port 143
    SMTP – port 25
    HTTP – port 80
    Secure SMTP (SSMTP) – port 465
    Secure IMAP (IMAP4-SSL) – port 585
    IMAP4 over SSL (IMAPS) – port 993
      Secure POP3 (SSL-POP) – port 995

Connection timed out Server replied: 110 Can’t open SMTP stream.

Sometime, when you try to send email using it would show following error.

Connection timed out
Server replied: 110 Can't open SMTP stream.

Or in outlook

could not retive

Usually, a workaround for this problem is to restart imap service on the server but it did not work so after a bit investigation i fixed the issue.
This problem usually occurs when CSF ( Config Server Firewall ) is installed on your server and it is interacting with the working of squirrel mail. If you would have enabled SMTP_BLOCK in the server and disabled SMTP_ALLOWLOCAL, a csf would not allow to send email from squirrel mail and would show error that i mentioned earlier.

When the option “SMTP_BLOCK” is enabled it is then required to enable the option “SMTP_ALLOWLOCAL” to allow the cPanel hosted Webmail programs to send e-mail; this includes Horde, SquirrelMail, and RoundCube, when accessed via cPanel Webmail. If the option “SMTP_ALLOWLOCAL” is disabled but the option “SMTP_BLOCK” is enabled then it will break the ability for the cPanel hosted Webmail programs to send e-mail.

$ vi /etc/csf/csf.conf


save and exit. Restart csf.

This will fix.

Client host rejected: cannot find your hostname

Usually, my blog posts are inspired from the client issues that we receive in a ticket. Today, I found customer stating that his emails are being rejected two providers in his area and has no problems sending emails out to anyone else (gmail, yahoo, hotmail, etc.) Quit interesting. Generally, as per my experience mail provider most of the problems we receive state that they have problem sending emails to gmail, hotmail etc but this one was different case.
In order to resolve this problem, I asked for the bounce back message that a customer receive and here is what I got.

The message identifier is: 1UxyzZ-0008U7-RN
The date of the message is: Sat, 13 Jul 2013 09:31:54 -0300
The subject of the message is: Proposta GaiaFP
The address to which the message has not yet been delivered is:
Delay reason: SMTP error from remote mail server after initial connection:
host [208.84.xx.xx]:
450 4.7.1 Client host rejected: cannot find your hostname, [192.xx.xx.xx]

Here you go, you found your error message.

450 4.7.1 Client host rejected: cannot find your hostname, [192.xx.xx.xx]

Hence, I ran rds lookup against the IP.

root@na []# host -t ptr

I noticed, rdns not being setup correctly which I setup later and fixed the problem!!

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