NoSpamProxy in detail

As the name NoSpamProxy already suggests, it is a pure SMTP proxy. Unlike conventional "store and forward" systems, the proxy only holds the part of the mail that is being transmitted at any specific point in time. There are no queues. This is important to know, in order to understand the principle and functionality of NoSpamProxy in full. NoSpamProxy starts by simply receiving the envelope data. In detail, this means that from HELO/EHLO through to the DATA command, everything is received and checked. At this point, several filters can already be deployed in order e.g. to check the sender IP, the mail address of the sender and of course the recipient's address for their validity, or the level of trust in them and possibly reject where appropriate. After NoSpamProxy has concluded its check, the envelope is transmitted to the nearest smarthost (inbound smarthost). At this point, the transmitting system has still not received any information about whether NoSpamProxy has accepted the envelope or not. It is only when NoSpamProxy has successfully transmitted the envelope to the inbound smarthost that it prompts the transmitting system to send the header and body data. Until this point in time, the transmitting system must wait for a response from NoSpamProxy. In normal cases, this process lasts for a fraction of a second.

The next stage is for the transmitting server to transmit the header and body. The inbound smarthost now has to wait. Should the transmission take longer, NoSpamProxy sends a keep alive signal at fixed intervals, so that the connection is not interrupted by a timeout. Once the header and body have been transmitted, they are also checked and evaluated by certain filters. If the evaluation is positive, the header and body are passed on to the inbound smarthost. As soon as this has accepted the complete mail, the transmitting system is also sent final confirmation. If the filter has evaluated the mail as bad, or the incoming smarthost will not accept the mail for any reason, NoSpamProxy will also not accept the mail and sends an error message to the transmitting server. The latter will thus not have fully transmitted its mail and has to create an NDR (non delivery report). The sender of the mail is immediately informed that his mail has not been received.

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