I maybe missing something really obvious here but my 3410 seems to be able to hold 10 txt messages in the inbox, after that it won't recieve anymore! This is really starting to annoy me, im on o2 pay as u go. I bet its blindinfly obvious isn't it. lol im so stupid.
As far as I know there's a maximum of 10. I used to have a 3310 and because it only has the SIM card's memory (no RAM). The SIM card already carries plenty of information so it limits you to 10 messages. You could try moving them to another folder but I think you're limited to 10 messages, period.
I've now got an 8210 - which I wouldn't recommend to my worst enemy as it's the least reliable thing I've ever owned - and the phone has memory of its own.