How to tell Gnus not to try to send mails / postings while I'm offline?
All you've got to do is to tell Gnus when you are online (plugged) and when you are offline (unplugged), the rest works automatically. You can toggle plugged/unplugged state by saying ‘J j’ in group buffer. To start Gnus unplugged say ‘M-x gnus-unplugged’ instead of ‘M-x gnus’. Note that for this to work, the agent must be active.