http://www.feep.net/sendmail/tutorial/intro/MUA-MTA-MDA.html

MUAs, MTAs, and MDAs There are three main parts of the Unix E-mail system: Part What It Does Examples Mail User Agent (MUA) The MUA is the program which the user uses to read and send e-mail. It reads incoming messages that have been delivered to the user's mailbox, and passes outgoing messages to an MTA for sending. elm, pine, mutt Mail Transfer Agent (MTA) The MTA basicly acts as a "mail router". It accepts a message passed to it by either an MUA or another MTA, decides based upon the message header which delivery method it should use, and then passes the message to the appropriate MDA for that delivery method. sendmail, postfix, qmail Mail Delivery Agent (MDA) The MDA accepts a piece of mail from an MTA and performs the actual delivery. mail.local, procmail The MTA is the most important one of these. It is responsible for doing all the "intelligent" work of e-mail transfer. While it does not actually perform any of the delivery itself, it is the part which tells the other parts how to interact and what to do. In a sense, the MTA is the glue which holds the whole process together. To illustrate how the three parts of the email system work together, here's a very general example. This is what happens when the user jsmith@host1.uiuc.edu sends e-mail to johndoe@host2.uiuc.edu: jsmith's MUA (pine, elm, etc) on host1.uiuc.edu passes the message to the MTA (sendmail) on the local host. The MTA (sendmail) notices that the message is addressed to a user at host2.uiuc.edu. Since it is configured to know that it can reach host2.uiuc.edu via SMTP, it passes the message to the SMTP MDA (the SMTP MDA is actually builtin to sendmail, but all other MDAs are external programs). The SMTP MDA connects to the MTA on host2.uiuc.edu (sendmail) and sends it the message. The MTA on host2.uiuc.edu (sendmail) notices that the message is addressed to a user on the local host, so it passes the message to the local MDA. The local MDA saves the message in user johndoe's mailbox. The next time johndoe logs in to host2.uiuc.edu and runs his MUA, the message is there waiting for him to read. As you can see, none of this would be possible without the MTA!

Breaking RCS locks.

Breaking RCS locks. So you want to edit some awesome file, only to see this when you check it out: co -l /mnt/yp/netgroup /mnt/yp/RCS/netgroup,v --> /mnt/yp/netgroup co: /mnt/yp/RCS/netgroup,v: Revision 1.1008 is already locked by u0064824. Blood and ashes! Here's how to break the lock so the world can move forward. First move the file to a backup location: mv /mnt/yp/netgroup /mnt/yp/netgroup-lockedByu0064824-Sep272002 Then break the lock rcs -u /mnt/yp/netgroup RCS file: RCS/netgroup,v Revision 1.1008 is already locked by u0064824. Do you want to break the lock? [ny](n): y State the reason for breaking the lock: (terminate with single '.' or end of file) >> Testing lock breaking -SH. >> . 1.1008 unlocked done Now you can check out the file and using diff (or vimdiff) you can see what changes the previous user made so you can hopefully merge in any changes they might have made. co -l /mnt/yp/netgroup diff /mnt/yp/netgroup /mnt/yp/netgroup-lockedByu0064824-Sep272002 Then edit away! When you're done we check the file in such that it stays in place and isn't magicly whisked away in the RCS repository: ci -u /mnt/yp/netgroup