Differences between CSSMTP and SMTPD

If you are currently using Communications Server SMTPD on z/OS®, you should consider using CSSMTP. CSSMTP has been designed to support z/OS users of SMTPD that create mail on the JES spool data set using batch jobs or that use SMTPNOTE for delivery to the Internet. The mail messages you send today using SMTPD can probably be sent with CSSMTP. However, as shown in Table 1, CSSMTP does have differences from SMTPD, and might not be able to accommodate everyone. Some of the primary differences that require migration consideration are as follows:

CSSMTP does not provide SMTP listener support; it cannot be used to receive mail to z/OS for delivery to TSO users. You must continue to use SMTPD for this kind of support or consider using sendmail for delivering mail to mailboxes defined in the z/OS UNIX file system.
CSSMTP delivers mail only to systems that have TCP/IP host names. If you are using a local TSO user ID on the POSTMASTER statement for your SMTPD server, then the local TSO user ID must be changed to an email address with the format userid@host.domain on the MailAdministrator statement for CSSMTP. For information about the MailAdministrator statement, see z/OS Communications Server: IP Configuration Reference.
CSSMTP does not perform DNS lookups to determine a target server for each recipient. CSSMTP requires a connection to an SMTP or ESMTP server that receives the mail and handles next-hop delivery. Typically, the target server is located at a remote destination, but could also be a local instance of sendmail.
If mail that CSSMTP sends to the target server becomes undeliverable, CSSMTP is not able to receive the undeliverable mail notification because CSSMTP is not a server. The originator (that is, MAIL FROM) of mail messages should be reachable from the target server.
CSSMTP processes a mail message directly from the JES spool data set and sends the mail message to the target server. If CSSMTP cannot deliver the mail message immediately, the message is held for a short period of time, defined by the RetryLimit statement. After this retry period expires, CSSMTP saves the mail message in the z/OS UNIX file system for the period of time defined by the ExtendedRetry statement. If CSSMTP cannot deliver the mail within any configured retry time limit, the mail becomes undeliverable. CSSMTP stores the mail in a z/OS UNIX directory between retry intervals.
CSSMTP does not support source routing, such as the <@host1,@host2:userid@host3> or <NJEuserid%NJEhost> formats. For example, in the address string @host1,@host2,...,@hostn:user_id@host_name, the @host1,@host2,...,@hostn portion of the address is ignored and user_id@host_name is used as the recipient value. If you are using source routing, you might want to change it to be an email address with the format userid@domain (mailbox). Otherwise, the mail might not be delivered.

Table 1. Differences between CSSMTP and SMTPD
Function	CSSMTP	SMTPD
Managing the JES spool file	Uses bad spool file or error report. For information about the BadSpoolDisp, Undeliverable, and Report statements, see z/OS Communications Server: IP Configuration Reference.	Uses DASD to store mail
Code page translation	Yes; uses iconv support of the EBCDIC code page for the JES spool file. For information about using the Translate statement, see z/OS Communications Server: IP Configuration Reference.	Yes; user-defined translate tables are configured in SMTPD
Modify RFC 822 headers or the body of the mail	No, except for code pages for converting from EBCDIC to ASCII, and for appending a RECEIVED line header to the beginning of the RFC 822 headers to indicate that CSSMTP has picked up the mail message.	Yes
Supports SMTP server function (for example, receive mail)	No, CSSMTP is a mail-forwarding SMTP client application and is not a fully capable MTA.	Yes
Deliver to local TSO user	No; if this function is required, use SMTPD to provide access to a local TSO mailbox.	Yes
Undeliverable mail	The undeliverable mail notification can either be sent back to the originator or deleted. An error report for undeliverable mail can be generated for SYSOUT or sent to the mail administrator. For information about the Undeliverable and Report statements, see z/OS Communications Server: IP Configuration Reference.	Send back to originator
Dead letter mail	The dead letter mail can either be deleted or written to a z/OS UNIX file system directory. For information about the DeadLetterAction parameter on the Undeliverable statement, see z/OS Communications Server: IP Configuration Reference.	No configuration parameters to support deadletter processing
Monitoring mail processing	MODIFY commands	SMSG
Requires VMCF and TNF subsystems	No	Yes; SMTPD is a Pascal application
Security	Uses security product authorization profile to provide security. See Security for CSSMTP	Uses the RESTRICT and SECURE statements
Rewrite the path name in command	No; The batch job needs to use a correct mail address format, such as userid@host.domain.	Yes
Addition of a message-id field when none appears	Yes	No
Source routing support, such as the @host1,@host2:userid@host3 or NJEuserid%NJEhost formats	No; CSSMTP removes source routes from the path. CSSMTP does not support source routing and does not generate these addressing formats.	Yes
Send mail to local mail administrator	No; CSSMTP internally generates a report that can be delivered to a mail administrator, but the mail administrator address must be in the form (userid@host.domain).	Yes; supports the local TSO or remote (NJE) user IDs as a mail administrator
Supports the SMTP commands EXPN, QUEU, HELP, NOOP, TICK, VERB, and VRFY	No; for commands supported, see z/OS Communications Server: IP User's Guide and Commands.	Yes
Resolver	z/OS UNIX search order for resolver configuration files. Uses system resolver.	Native MVS™ search order for resolver configuration files. Does not use system resolver.
Checkpoint	Yes; for information about using the CHKPOINT DD statement in the CSSMTP started procedure and about starting CSSMTP, see z/OS Communications Server: IP Configuration Reference.	No
IPv6 support	Yes	No
SMF support	Yes	No