Communications Server Simple Mail Transfer Protocol (CSSMTP) is a mail-forwarding SMTP client application. CSSMTP processes data sets containing mail messages on the spool file and forwards them to a target message transfer agent (MTA) without resolving each recipient. If you currently use MVS™ batch jobs to have the SMTP server (SMTPD) send bulk mail that does not typically require a reply to the originator, you can use CSSMTP to process this mail. CSSMTP can improve the performance, scalability, and availability of the client function of SMTPD, but it does not act as a listening MTA server like SMTPD. CSSMTP can coexist with SMTPD, and multiple instances of CSSMTP can run on a single host. Figure 1 shows how CSSMTP fits into a network.
CSSMTP implements RFC 2821 and RFC 2822 for interacting with server MTAs, and supports additional RFCs for message size (RFC 1870) and security (RFC 3207). CSSMTP is not a fully capable MTA and functions as an outbound forwarder, sending mail messages from the JES spool data set to the Internet. As the mail messages are read from the spool file, CSSMTP functions like a TCP/IP SMTP client and interacts with one or more configured target servers. When processing mail messages from the spool file, CSSMTP does not resolve mail message recipients, but transfers mail messages to one or more configured, next-hop servers (fully capable MTAs) that might or might not be the final destination.
CSSMTP provides the following capabilities:
- Checkpoint capabilities
If you need to restart CSSMTP, it does not have to reprocess the entire spool file, which will reduce duplicate mail that is received by message recipients.
- Long retry and extended retry capabilities
- The long retry capability is available for up to 5 days, and is intended to compensate for short-term target server outages.
- The extended retry capability is available for a longer period or for an indefinite period, and is intended to compensate for extended target server outages.
- Multiple security capabilities
See Security for CSSMTP.
- SMF recording of CSSMTP events
See Steps for configuring SMF records for CSSMTP (optional).
As stated in RFC 2821, SMTP clients that transfer all traffic, regardless of the target domain names that are associated with the individual mail messages, or that do not maintain queues for trying mail message transmissions again that initially cannot be completed, might otherwise conform to this specification but are not considered fully capable. CSSMTP does not implement all aspects of RFC 2821.
If you are moving from SMTPD to CSSMTP, see Differences between CSSMTP and SMTPD.