When a topic gets locked, it is indicated in at least two ways on the web site (see screenshot attached), and in addition, a moderator can send a final reply to tell everyone that the topic is locked.
This final reply is also somewhat useful for e-mail users, as long as they see the final reply before they reply to any of the other messages in the locked thread.
[However, whether the final message in a topic appears at the very bottom of the thread depends on the e-mail program, and not everyone reads all newly arrived mails before they press SEND on the message that they've been writing.]
I would like to suggest three ways in which it can be made clearer to e-mail users that a topic is locked.
1. Create a member notice called "Locked Topic", so that moderators can save time when writing a "this topic is now locked" message.
2. When a moderator locks a topic, he should be given the option to send a message (including the "locked topic" member notice) to the group that the topic is now locked.
The notice should have the *same* subject line as the topic (so that remains part of the topic), but with [locked] added somewhere in the subject line (before the group name, after the group name, etc.) so that e-mail users can see that there exists a message with the "locked" label in that thread's list of messages.
3. When a moderator responds in a topic, he should have the option to specify "lock topic" before the posts his message, and if he does so, the [locked] label should also be added to the subject line.
(Instead of [locked], one can also use #locked, but that would be inconsistent with how hashtags currently work and might lead some moderators to mistakenly "learn" that they can lock a topic with a hashtag, which is not what I'm suggesting.)
It so happens that I know that "Mark Fletcher" is the list dad for this list, so when I see a reply by Mark, I read it first, but list users do not always know who the moderators are and may not open the moderator's message to discover that he had locked the topic.
I'm not sure if members who senda message to a locked topic get a notification (I didn't, in my tests), but it is disheartening to users to have carefully crafted a well-written, properly researched message, only to discover later that the message disappeared into the ether (or eventually discovering that the topic was locked and that they had wasted their time) -- such users learn the hard way that being quick is better than being considerate.