The CeSMTPD is composed of three execution files and two configuration files.

Sparrow and pigeon's operation is decided by the configuration file.

Base specifications

Sparrow and pigeon's specifications conformed to RFC2821 and RFC1939.  The following besides a basic specification are implemented.

Especially, we will recommend SMTP-AUTH and POP before SMTP to be used to prevent the SPAM relay.
It is not significant to handle both at the same time. Because the password is the same.

You should deny an external mail request from the host who doesn't know basically.

Recipient restrictions

Because the installation of CeSMTPD is easy, the default setting is set to the safety side.  It is likely to have a hard time a little to transmit the mail to the outside domain.

When the Sparrow is requested sending mail, the sparrow decide if the mail can send according to the following flowchart.

The rcpthosts.txt file is needed in the beginning.
If the domain of recipient address has been in rcpthosts.txt, the delivery is unconditionally tried. It may be useful in the mail forwarding among Intranet.

Next , Sparrow compare with the mail's domain and 'me'.

3rd, if 'POP before SMTP' option is on, Sparrow searches for the footprint of the POP3 authentication.
if the option is off and SMTP-AUTH option is on, Sparrow require the SMTP-AUTH to client.


CeSMTPD is implemented to which bounce doesn't do easily unlike a lot of SMTP
servers.  Because after a local delivery ends, 250 is returned when the RCPT command is received. It is moreover because 'POP before SMTP' and 'SMTP-AUTH' are done by default.

When SMTP is relayed, many of bounce mail is generated.
Of course, CeSMTPD relays the bounce mail when relaying it as shown in the figure

(Local SMTP: CeSMTPD / Remote SMTP: relayed SMTP)

CeSMTPD doesn't do a triple bounce. A triple bounce is to forward the Dablbaunsed mail to another account again like the figure below.