Outbound message tasks are then responsible for delivering the message to the Internet. There are two methods of transfer. If the SMTP MTA has been configured to send mail to a relay host, an outbound session task transfers the message to that host.

However, if a relay host is not in use, the SMTP MTA must perform a lookup of the destination host's address by querying Domain Name Services (DNS) to find the destination host's address. Once the address is obtained, the outbound session task establishes a session with the destination host, and the SMTP MTA transfers the message to the destination host.

Messages coming into the SMTP MTA from external SMTP sources are handled in somewhat the same manner, but in reverse order. SMTP servers listen for incoming mail on port 25. This is the standard SMTP port throughout the Internet so that all SMTP servers do not have to spend time negotiating how they will communicate.

Inbound messages are first received by the SMTP MTA's inbound session controller task which listens on port 25. A new session is created as inbound messages are received. The inbound message is transferred into the SMTP inbound work queue database in SMTP format. A SMTP MTA inbound conversion task monitors this database, and as new messages come into the database, they are converted to Notes format by an inbound message conversion task, then delivered to the smtp.box database.

A summary of transfer steps
Whew! To summarize, the following steps are performed when sending SMTP mail:

• Mail is routed to the server which is running the SMTP MTA;

• The SMTP MTA converts the message from the native format to SMTP and MIME;

• SMTP MTA either delivers the mail directly to a mail relay host or the SMTP MTA performs a lookup of the destination host's address using DNS;

• The SMTP MTA connects to the destination host if available and then delivers the message.

And, to summarize, the following steps are performed when receiving SMTP mail:

• The SMTP MTA listens on port 25;

• When a message is received, a new session is created;

• The incoming mail message is stored within the inbound work queue in SMTP format;

• SMTP MTA converts the message to native Notes format.

Dealing with problems
As described, several tasks take several sequential steps and use several databases to provide SMTP services. Unfortunately, this means there can be several possible points of failure.

Figure A shows show a screen shot of the outbound work queue database. Messages are sorted by status type. Of particular interest is the message under the status "WQ_DEAD_STATE". These are "dead" messages; messages which will not be routed.

FIGURE A


The messages under WQ_DEAD_STATE couldn't be delivered. Roll over picture for a larger image.

There are a huge number of reasons for the existence of dead messages. Dead messages may also be caused by delivery failures, invalid responses by a SMTP server during SMTP communications, invalid message headers, or by an inbound messages having no sender listed in the "from" field. One of the more common reasons for dead messages is missing or invalid Person documents.