In order to combat spam, avoid receiving complaints (abuse) and getting hosting addresses into various blacklists, we have to control outgoing connections to TCP port 25 (SMTP) on the border router.

In case of an abnormal increase in outgoing SMTP traffic on the TCP / 25 port, the sender's IP address is blocked for a period of 1 day.

Normal MTA mail servers with nominal load, which do not send SPAM, are not covered by this filter.

To be able to send mail messages from virtual and physical servers of the RealHOST hosting to the Internet, you can also connect an SMTP-relay provider. For this:

  • on the server, configure MTA to forward mail through the host port 25

for example, for postfix MTA, add the following line in /etc/postfix/ file:

relayhost =

  • add (or correct) an SPF record to the domain zone for all domains whose mail will be forwarded through so that the record contains the line, for example,

v=spf1 a mx ~all

SMTP relay has standard restrictions on mail forwarding:

  • Maximum mail message size - 30 MB
  • Number of messages sent from one IP address within 1 minute - no more than 60
  • Number of messages sent from one IP address per day - no more than 1500