Yesterday I tried to send a sms and the outbox reported that the sending failed. Tried several times with the same result. I then changed my default internet setting from RAS (using a direct cable connection) to another connection that uses the gsm dataport and the sms was send without any further problems.
Are you talking about SMS or Email? If you have problems with sending SMS, check your short message settings, maybe there was something wrong at your service centre.
You can as well temporary move your schedules.dat file from C:\system\schedules and then try sending it again.