My messages sometimes take hours or days to show up on the list!
Most top-level lists are called "discuss" lists. This means that only people who are subscribed can freely post messages. Messages sent to a discuss list from an email address that is not subscribed are bounced to a moderator for approval. If the message is valid a moderator will approve it and it will be delivered to the rest of the list.
This process helps cut down spam and viruses coming from random addresses. However the moderator (or moderators, as there is often a team) are only human, and must read and evaluate every message that is bounced to them. Depending on what time zone you are in, there might not be a moderator active when you send your message, so it might sit in the queue, awaiting attention, for a few hours. Please don't re-post your message--the same thing will happen to the second message, and they might both eventually get delivered to the list!
To avoid your messages being moderated, make sure you are subscribed to the list. Remember that you must post your messages from the same address that is subscribed, for exmaple, if you subscribe with address1 but then post messages from address2, your messages will be moderated.
If you'd like to help moderate for a few minutes a day, see here .