TETENAL I believe this is an OS X issue and not just a Mail problem, but if you feel it needs moving to the Applications forum please go ahead!
My friend called me last night with a problem with Mail on his G4. After typing for a few seconds in a new mail message Mail stops responding until you click in the mail message window with the mouse again.
After much investigating around Mail ( permission repairs, reinstalling 10.4.7 combo updater, swapping a known copy of Mail, trashing Mail prefs ), I noticed that Mail stops responding when you type a 'space' or other punctuation (took me a while I admit, but it was late!) . A quick trip to Mail->Prefs and turning off 'check spelling as you type' and you can type for as long as you like, BUT Mail will not send the message, it ignores the send button - sigh!
So in Mail I went to Edit->Spelling->Spelling… to change the spelling settings but nothing happens when you select this menu, nothing no error, nothing.
This prompted me to check wider, the same problem in iChat and Safari.
So I fired–up Dictionary and it works fine. But if you select a word in another app and select 'Look up in Dictionary' from the contextual menu nothing happens…Unless you have 'Open Dictionary application' selected not 'Open Dictionary panel' in the Dictionary preferences AND you already have the Dictionary application open, then the definition appears in the Dictionary window!
I have tried removing any prefs that could do with Dictionary, investigated 'Application Support' All no no avail. I cannot find the elusive corrupted preference or plug-in or support file or what ever that might me causing this.
One last thing… using the normal 'create a new user' to test his local settings rather than the general system, showed that the problem was local to him. The new user worked fine.
So I have concentrated on his home directory with my useless troubleshooting!