1. 24x7x365 TCP/IP connectivity, with enough bandwidth to handle your workload.  Please note carefully that LISTSERV is not designed to dial up and access POP mailboxes. The proper installation and operation of LISTSERV is contingent on direct connection to the Internet in order for it to receive and send SMTP mail.  If you do not understand this, L-Soft strongly recommends that you get expert technical advice before proceeding further, or consider using a hosting service instead of running LISTSERV yourself (for an overview of hosting services offered by L-Soft, please see  https://lsoft.com/products/listplex.asp.
    2. In conjunction with networking requirement #1, and as a fundamental requirement of the SMTP mail protocol, LISTSERV requires a static IP address which is mapped via a DNS A or MX record to a particular host name (typically LISTSERV.yourdomain, for example, LISTSERV.EXAMPLE.COM). If you are planning to use the web archive/administration interface with LISTSERV, an A record is required at minimum (web browsers cannot connect to hosts that have only an MX record).  Please note carefully that L-Soft does not support LISTSERV servers running without DNS entries (i.e., we do not support servers configured with bracketed IP addresses instead of FQDNs [fully-qualified domain names, for example, LISTSERV.EXAMPLE.COM]).

Note the following carefully:

    • The use of a CNAME is STRONGLY DISCOURAGED because typically such hostnames are rewritten ("canonicalized") when mail sent from them traverses the Internet.  This will cause mail to appear to be coming from a "different" host than your subscribers may be expecting, even though in fact the A and CNAME records point to the same physical machine.
    • The modern Internet mail system is becoming more and more heavily dependent on DNS-based anti-spam protocols such as DMARC, DKIM, and SPF.  LISTSERV is designed to support these protocols and is capable of signing outbound email with DKIM (which is one of the keystones of the DMARC system).  For more information, please contact the support department, or see our Advanced Topics Manual, which goes into more depth on these subjects.
    • Microsoft's WINS and MAPI protocols are not supported or used by LISTSERV.
    • There are no guarantees that LISTSERV will work with a dynamically-assigned IP address (for instance, one obtained through DHCP), and a DNS entry is required if you expect people to be able to send mail to, and also--in many cases where the user's ISP does a reverse DNS lookup to validate that the mail is not spam--to receive mail from the server.

       Again, if you do not understand any part of this requirement, L-Soft strongly recommends that you get expert technical advice before proceeding further.

    1. If installing LISTSERV behind a firewall with the expectation that users from the outside world will be able to access it, it is imperative that you provide access to the following TCP ports on the LISTSERV machine:  Port 25 (SMTP) and port 80 (webserver, if the web archive/administration feature is enabled).  It may also be necessary to provide access to port 53 (nameserver) if LISTSERV is unable to resolve the name of its outgoing mail host.  Opening other ports may be necessary for other features to work properly (e.g., port 443 if your web server supports SSL/https).

Installing LISTSERV behind a firewall is almost more of an art than a science (and every firewall seems to be different) but the bottom line is that the above ports must be open if you expect LISTSERV to work.  It is not recommended to route incoming LISTSERV mail through another machine (i.e., the firewall) as this normally leads to complications.