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Questions tagged [smtp]

SMTP is the Simple Mail Transfer Protocol, the most recent standard of which is defined in RFC 5321. It's the Internet standard for sending and receiving email.

Some characteristics of SMTP (Simple Mail Transfer Protocol):

  • The default port is TCP port 25 and new submissions (MSA) is port 587.
  • Although depreciated, some services still use port 465 (SMTPS) to support legacy applications.
  • It is used to transmit outbound mail, and mail servers (which listens on port 25) receives mail on port 25.
  • It should not be confused with POP3 / IMAP which is an actual store for the e-mail. SMTP messages are received by the receiving server and then an MTA moves it to the POP3 / IMAP mailbox where it is normally retrieved using a POP3 / IMAP client.

See Also:

RFC5321

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Postfix reject_unknown_reverse_client_hostname: replace default unknown_client_reject_code (450) to 550. Why/When I should not?

In the daily battle against SPAM, there have been several times when I've been tempted to heavily enforce DNS requirements from clients connecting from the wild Internet. In detail, I would have added the reject_unknown_reverse_client_hostname…
Damiano Verzulli
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postfix multiple IP SMTP banner

I've installed postfix 2.11.3 and we have multiple IP addresses on our server which we want to host two domains, isolated and running on their own IPs. domain1 unix - - n - - smtp -o smtp_bind_address=1.1.1.1 -o…
Satish
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What's the best way to check if an SMTP server is SSL-enabled or not?

What's the best way to check if an SMTP server is SSL-enabled or not? Follow-up question: How do I make it SSL-enabled if it's not yet SSL-enabled. The OS is CentOS.
Randell
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Why is email being delivered normally despite an SPF "hardfail"?

I'm trying to figure out why forged email is being delivered to major email providers (gmail.com, outlook.com) even though email is marked with an SPF hardfail. The email is also delivered to Microsoft Exchange, which is throwing a PermError for…
Rook
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Recommendations for SMTP services for massive mailing

What is the recommendations for SMTP services for massive mailing? Let me clarify, most of the free SMTP services (Gmail, Yahoo, Hotmail, etc...) has limits about the amount of messages you can send in a day. Gmail, for instance, only allows you to…
Daniel Silveira
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Fastest way to allow sendmail relay through smarthost?

I only want sendmail to relay through my ISP's smtp server, nothing else. How do i set it up?
Carl Hörberg
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Does SMTP greylisting a) stop much spam and b) stop much legitimate mail?

I've just set up an SMTP server on a relatively little used domain using Postfix and enabled greylisting with SQLGrey. So far it seems to be working OK, and while there's the slight irritation of delays to emails from new senders, I can see from…
Whisk
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Should I use SSL for a receive only SMTP server?

I am creating an smtp server which will not need to send emails but only receive. Not being an expert, I was wondering if I should still use an SSL certificate for security. For example, if I send an e-mail message from my outlook or gmail client…
Sauron
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How do I prevent the SPF_HELO_NONE warning when sending from Postfix?

When using a tool like https://dkimvalidator.com/ to verify configuration of DKIM, SPF, DMARC, etc. for sending mail from a web server, I get a warning like this: 0.0 SPF_HELO_NONE SPF: HELO does not publish an SPF Record It seems like a bad…
Walf
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How important are PTR records for Email Servers?

Does anyone know of any studies done to show how much email will be rejected if there is not ptr record for the SMTP banner name of an email server? Are reverse checks always done when enabled, or is it sometimes configured so if an spam program…
Kyle Brandt
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Postfix What does "Queued mail for delivery" mean?

I sent an email to 2 receipients and I checked maillog: tail -5000 maillog | grep "020EE4202065" Aug 7 17:31:24 mail postfix/smtpd[7478]: 020EE4202065: client=NodoOffice[118.70.28.223], sasl_method=PLAIN,…
ThanhLoyal
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How can I use wildcards for sendmail TLS_Rcpt?

sendmail lets one place restrictions on TLS conversations. I want to check that messages sent to example.com are sent to a server that has a *.messagelabs.com certificate. I want to protect against DNS spoofing and MitM. If messagelabs only had one…
Law29
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Windows Server 2012: Cannot install SMTP-Server, system always requires restart

I'm not able to install the "SMTP-Server" feature on a Windows 2012 server. The installation process always fails with a "restart required" message. After the restart and (re)starting the SMTP-Server feature, the system requires a new restart. I…
janjonas
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Why would an SMTP client send a RSET (Reset) command?

I’m trying to understand how exactly an email is processed by Postfix – and some of the finer details of SMTP mail transactions. My short-term goal is to debug a proprietary (binary, closed source) SMTP client but I first thought I’d examine what…
Anthony Geoghegan
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postfix fails to send mail with "fatal: unknown service: smtp/tcp"

I have installed postfix, and when I tried to send a mail to gmail it told me that it has been sent, but when I checked my inbox in Gmail I didn't saw it. This is how the connection looked like: $ telnet localhost 25 Trying ::1... Connected to…
LessComplexity
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