Mailing list managers (Listserv, Mailman) vs MTAs (Sendmail, Postfix)

Some terminology:
Mail Transfer Agent: MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol.
An MTA receives a message from another MTA, MSA or MUA. If the recipient mailbox is not hosted locally, then it routes it to another MTA. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.
MX: A mail exchanger record (MX record) is a type of resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain and a preference value used to prioritize mail delivery.
MUA: Mail user agent - this is an email client like GMail, Outlook etc. MUA uses POP3 (Post Office Protocol) or IMAP (Internet Message Access Protocol) to retrieve messages from an MTA.
MSA: Mail submission agent - sits between the MUA and MTA. Functionally same as MTA.

Sendmail/Postfix use information from the Domain Name System (DNS) to figure out which IP addresses go with which mailboxes.
1. Setup a domain name -e.g. companyA.com
2. Configure name servers for your domain (primary and secondary)
3. Configure MX records for your domain.
4. After the name servers are setup, register your domain using one of the registries.
5. Configure sendmail to listen for mail/route outgoing mail.
6. Mailing lists are configured by setting up aliases. In Postfix, edit the /etc/aliases file. It has the format:
alias: address1,address2. After updating this file, you are usually required to run commands to update the internal db file used by Postfix/Sendmail.

Mailing list managers like Mailman integrate with an MTA like Sendmail or Postfix, so that when new lists are created, or lists are removed, Postfix's alias database will be automatically updated.