1. SMTP: $ tcpip set conf smtp/eightbit/relay/top_headers - /gateway=Alternate= - /Substitute_domain=NAME= - This makes SMTP on my system show as: $ TCPIP SHO CONFIG SMTP SMTP Configuration Options Initial interval: 0 00:30:00.00 Address_max: 16 EIGHT_BIT Retry interval: 0 01:00:00.00 Hop_count_max: 16 RELAY Maximum interval: 3 00:00:00.00 TOP_HEADERS Timeout Initial Mail Receipt Data Terminate Send: 5 5 5 3 10 Receive: 5 Alternate gateway: xxxxxx.provider.com General gateway: not defined Substitute domain: HIDDEN, domain.tlb Zone: domain.tlb Postmaster: TCPIP$SMTP Log file: SYS$SPECIFIC:[TCPIP$SMTP]TCPIP$SMTP_LOGFILE.LOG Generic queue Queues Participating nodes TCPIP$SMTP_node_00 1 Node $ My mailserver MUST be a relay host (other machines in the network relay via this host) and my provider REQUIRES me to take measurement to prevent it to become an open relay. You do this by changing SYS$SPECIFIC:[TCPIP$SMTP]SMTP.CONFIG. This file contains a full description! You can do a lot of things, but THESE do what the ISP requests. First. limit INTERNAL access to the machines that can use this system as relay. Other macines will NOT be allowed to contact. You can do that by name (fully qualified) or by network: Good-Clients: , Second, define for which domains this server can act as relay. Obviously, it's your own, but all subdomains as well! You can host other domains as well but you need to specify them explicitly. Relay-Zones: , Third: To prevent spam - at least, part of it - accept mail from a limited number of domains: that is: your own but others as well (Can be a requirement if the name cannot be translated into a domain name using the world's DNS. You need to do that internally (your interbnal DNS or hostfile(s)) Accept-Mail-From: *@, *@, *@ There are some measurements you can do there as well but I think these are off topic.