Test Dovecot IMAP Server with "telnet" Client

Email Tutorials - Herong's Tutorial Examples

∟Test Dovecot IMAP Server with "telnet" Client

This section provides a tutorial example on how to test Dovecot IMAP server with the 'telnet' client command to send a POP3 request to port 143 on local host. Users can retrieve emails after authenticated with their passwords.

One way to test the Dovecot server is to use the "telnet" client command. All we need is the IP address of the server and the IMAP port number, which is defaulted to 143. Here is how I did it.

1. Run the following "telnet" session on my CentOS computer where Dovecot server is running.

herong$ telnet localhost 143
  Trying ::1...
  Connected to localhost.
  Escape character is '^]'.
  * OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE
    LITERAL+ STARTTLS AUTH=PLAIN AUTH=LOGIN] Dovecot ready.

2. Login directly since "PLAIN" authentication is supported. Note that the first token of the command is a custom label for the command.

c1 login herong TopSecret
  c1 OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE
  SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT
  MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS
  LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES
  WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE SNIPPET=FUZZY
  PREVIEW=FUZZY LITERAL+ NOTIFY SPECIAL-USE] Logged in

3. Select a mailbox folder. The default is "INBOX". I see 2 emails in the folder.

c2 select "INBOX"
  * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
  * OK [PERMANENTFLAGS (\Answered \Flagged \Deleted \Seen \Draft \*)]
    Flags permitted.
  * 2 EXISTS
  * 0 RECENT
  * OK [UIDVALIDITY 1605272831] UIDs valid
  * OK [UIDNEXT 22] Predicted next UID
  c2 OK [READ-WRITE] Select completed (0.001 + 0.000 secs).

4. Retrieve the first email.

c3 fetch 1 RFC822
  * 1 FETCH (RFC822 {416}
  Return-Path: <herong@example.com>
  X-Original-To: herong
  Delivered-To: herong@example.com
  Received: from localhost (localhost [IPv6:::1])
    by mail.herongyang.com (Postfix) with ESMTP id 5EABD266002B
    for <herong>; 15 Nov 2023 06:19:34 -0500 (EST)
  Message-Id: <20231113111946.5EABD266002B@mail.herongyang.com>
  Date: 15 Nov 2023 06:19:34 -0500 (EST)
  From: herong@example.com

  Hello world!
  )
  c3 OK Fetch completed (0.022 + 0.000 + 0.021 secs).

5. Close session and logout.

c4 close
  c4 OK Close completed (0.001 + 0.000 secs).

c5 logout
  * BYE Logging out
  c5 OK Logout completed (0.001 + 175.703 + 175.702 secs).
  Connection closed by foreign host.

6. Open firewall to handle remote IMAP requests on port 143.

herong$ sudo firewall-cmd --zone=public --add-service=imap
herong$ sudo firewall-cmd --runtime-to-permanent

7. Run the following "telnet" session on my macOS computer which is connected to the CentOS computer. The output shows that the Dovecot IMAP server is configured to require SSL/TLS connections from remote computers.

herong$ telnet 192.168.1.100 143
  Trying 192.168.1.100...
  Connected to 192.168.1.100.
  Escape character is '^]'.
  * OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE
    LITERAL+ STARTTLS LOGINDISABLED] Dovecot ready.

c1 login herong TopSecret
  * BAD [ALERT] Plaintext authentication not allowed without SSL/TLS,
    but your client did it anyway. If anyone was listening,
    the password was exposed.
  c1 NO [PRIVACYREQUIRED] Plaintext authentication disallowed on
  non-secure (SSL/TLS) connections.

c2 logout
  * BYE Logging out
  c2 OK Logout completed.
  Connection closed by foreign host.

Ok. Dovecot IMAP server works for local clients and requires SSL/TLS connections for remote clients. See next chapter on how to test SSL/TLS connections with "openssl s_client" commands.

By the way using plain (clear text) user name and password in remote connection is extremely bad, since anyone on the network can catch the communication data packet and see your password.

As a reference, here a list of commonly used IMAP commands. All commands are case insensitive.

xx login <username> <password> - Log in to the IMAP server with the given username and password.
xx capability - Return a list of capabilities supported in the current context.
xx namespace - Return the prefix and hierarchy delimiter to the current personal namespaces.
xx list <base> <path> - List mailboxes that match the given path under the given base. Example: 'xx list "" *'.
xx select <mailbox> - Select the given mailbox as the current mailbox. Example: "select inbox". "inbox" is the default mailbox name for most IMAP servers.
xx examine <mailbox> - Select the given mailbox as the current mailbox in read-only mode. Example: "select inbox". "inbox" is the default mailbox name for most IMAP servers.
xx fetch <index> <format> - Fetch the message of the given index in the current mailbox in the given format. Example: "fetch 1 rfc822".
xx copy <index> <mailbox> - Copy the message of the given index from the current mailbox to the given mailbox. For example: "xx copy 3 Spam".
xx search <field> <criteria> - Search messages with the given criteria in the given field in the current mailbox. Example: "search from herong"
xx store lt;index> +flags (lt;flag>) - Mark the message of the given index with the given flag. For example: "xx store 2 +flags (Deleted)".
xx expunge - Delete messages that marked as "\Deleted" in the current mailbox.
xx close - Close the current mailbox. Messages marked as "\Deleted" will be deleted from the server.
xx create <mailbox> - Create a new mailbox. Example: "xx create Junk".
xx rename <old> <new> - Rename a given mailbox. Example: "xx rename Junk Spam".
xx delete <mailbox> - Create a given mailbox. Example: "xx delete Spam".
xx status <mailbox> (<status_list>) - Return counts of messages that match given statuses. For example: "xx status inbox (messages recent)".
xx logout - Close the connect. Messages marked as "\Deleted" will be deleted from the server.

Table of Contents

About This Book

Introduction to Email

Postfix - Mail Transport Agent (MTA)

SSL/TLS Secure Connections with Postfix Server

►Dovecot - IMAP and POP3 Server

What Is Dovecot

Install and Configure Dovecot on CentOS

►Test Dovecot IMAP Server with "telnet" Client

Test Dovecot POP3 Server with "telnet" Client

Keep Deleted Emails on Dovecot

Turn on Dovecot Server Logging for Troubleshooting

"doveadm" Command - Dovecot's Administration Utility

SSL/TLS Secure Connections with Dovecot Server

Email Client Tools - Mail User Agents (MUA)

Mozilla Thunderbird - Mail User Agents (MUA)

References

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