I have a N900 and today, whenever I send an SMS (to ANYBODY), as soon as I click on SEND I get:

SMS Delivery Failure for XXX


this hapens pretty much as soon as I press SEND, suggesting somethings up with the SMS message centre settings on T-Mobile for the N900.
The weird thing is, that I haven't done anything with the settings, it just refuses to send SMS's to anybody. The Phone works fine, so does the Internet on 2G and 3G.

Any suggestions?

Check your SMS center number, conversation>settings, if it's blank check your provders website to get the number.

Call your service provider regarding the problem

I once had a similar issue, but in my case the whole telephony stuff gone.

Once I rebooted the n900 it worked since now.

My SMS center setting simply says "From SIM". And it works.

strange...funny how the data connection works and i can make calls, but cant send an sms..even after reboot. hmmmmm......

I get a different issue,

When I send a text it will say delivered, but in the Conversations it will show a red triangle with white exclamation mark next to a contact which indicates the text failed. However, they do get the text.

I don't get how this issue has just happened...I.e. I've owned the device on T-Mobile for about 2 weeks, and all of a sudden I can't send sms's to ANYONE neither will my Exchange connect.
It's bugging me like mad!! Espicially as ALL other data modes work (i.e. 3G data). Weird.

OK as an update my Apps Catalogs are gone from Apps and Uninstalls reports no Apps installed !!! Time for a reflashing. definetly not a phone for a noob but hey, its irresistable; even if somewhat tempramental.

@Doc_Raj: Open an xterm and do
(it's much easier to write if you use ctrl-i (i.e. TAB) a lot, to save keystrokes.)
If that command doesn't turn up empty, or with an error, then just reboot and there's a good chance you'll have your apps catalogs back.

Of course, there may be other reasons for reflashing anyway, like your SMS trouble.