Groups of posts are called "digests" in Mailman. Rather than get messages one at a time, you can get messages grouped together. On a moderately busy list, this typically means you get one email per day, although it may be more or less frequent depending upon the list.
You may also want to look at Section 8.2 which discusses MIME and plain text digests.
To turn digest mode on or off using the web interface,
Set it to "On" to receive messages bundled together in digests. Set it to "Off" to receive posts separately.
To turn digest mode on or off using the email interface,
Commands can appear in either the body or the subject of the message. (See Section 3.2 for more information about sending mail commands.)