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how_to:incoming_smtp_feed

Incoming SMTP Feed

An incoming SMTP feed is where incoming mail is sent directly to your mail server without going through your ISP's POP3 mail server on the way.

Advantages

An incoming SMTP feed has several advantages over going via a POP3 mailbox:

  • Incoming BCCd messages work correctly without any effort
  • Incoming mail arrives immediately at your mail server, without any delay
  • Any restrictions, quotas, limits etc placed by your ISP are bypassed
  • VPOP3 can handle several incoming messages at once, unlike with POP3 collection where it will only download one message at once.

Disadvantages/Requirements

However there are a few gotchas to watch out for:

  • You must have your own email domain (the part of your email address after the @ sign).
  • You must have a permanent IP address on the Internet for incoming SMTP to work. Other mail servers have to know where to send your messages, so if your IP address can change, they will lose track of where to send the messages
  • You must have a permanent Internet connection. It doesn't matter if it may go down for a few minutes, but it won't work with a dial-up connection unless you are almost permanently dialed in. The senders decide when to send mail to you, you don't decide when to collect it.
  • Your VPOP3 server must be running all the time. Again it doesn't matter if it is down for a few minutes (or even hours) for maintenance, but this shouldn't be the norm - so you can't turn the VPOP3 PC off at night.
  • You have to allow incoming SMTP connections through your firewall (port 25 TCP) to the VPOP3 computer
  • Your ISP has to allow incoming SMTP connections to you on port 25. A few ISPs will block port 25 by default, and require you to ask for it to be opened up. (A very few ISPs won't open it up even if you ask). This is a security thing, as badly configured mail servers can be used by spammers.
  • You have to make sure that you don't configure your VPOP3 server as an open relay. By default VPOP3 will not be set up as an open relay, but if you change the SMTP service IP Access Restrictions without understanding what you are doing you may make it into an open relay - that will allow spammers to send millions of messages through your mail server without your permission.
  • VPOP3's SMTP Rules for incoming SMTP are slightly less powerful than the Download Rules. This is due to differences in the way the two mechanisms work - for instance you cannot have VPOP3 ask you if you want to download large messages, because that is simply not possible.
  • With incoming SMTP mail, the mail is not stored somewhere else as well. With incoming POP3, you can tell VPOP3 to leave messages on your ISP for a few days as a safety net. This is simply not possible with SMTP, as there is no ISP involved.
  • You cannot use SSL session encryption on port 25 if you want incoming SMTP.

With the above restrictions, if your Internet connection or mail server goes down for a short time, it is usually no problem at all. Well configured mail servers will queue and retry messages which failed to send, usually for up to 3 days, because the Internet is never 100% reliable. However if it is possible that your mail server may be down for more than 3 days, then you should consider either not using SMTP, or using a backup SMTP mail service which can take over if your main mail server fails.

How to do it

There is usually nothing you need to do in VPOP3, as it is always listening out for incoming SMTP connections.

Usually all you need to do is:

  1. Open up incoming TCP port 25 connections on your firewall, and direct them to the VPOP3 computer.
  2. Change the DNS MX Record for your domain to point to a host name which resolves to your permanent IP address.

That's it

  1. Ensure that the Local Domains setting (in Settings → Local Mail → General) contains your domain name(s) - if there are multiple domains, separate them with semi-colon characters (';')

If you have changed the VPOP3 settings from the defaults, then you may need to make some changes. Go to Services → SMTP, then:

  • Check Do not require SMTP authentication for internal/incoming mail
  • Check Reject unrecognised local recipients
  • Make sure the SMTP Anti-Relay Protection method is set to Check Client IP Address
  • In the IP Access Restrictions tab, make sure it has your local network address in there as allowed, and do NOT have an Allow Any row in the restrictions unless you want to allow remote users to send outgoing mail through your VPOP3 server. If in doubt, press the Default for your LAN button.

When you have the SMTP Anti-Relay Protection method set to Check Client IP Address the Access Restrictions tell VPOP3 which mail servers can send outgoing mail through VPOP3 - all computers on the Internet can send mail TO VPOP3 users, so incoming mail will work fine, even if it looks as if the sending computer address may be blocked.

If you have a support contract with us, we will be glad to help you with this, as it can sound complicated if you do not have experience in doing this. Alternatively you can purchase a technical support incident (or take out a contract) and we wil be able to help you, including setting it up remotely for you.

how_to/incoming_smtp_feed.txt · Last modified: 2018/11/14 10:45 (external edit)