Exact same issue here; I looked at the Queue just before I went home and there were 190 odd messages waiting to go out. I first did a telnet session to our smarthost with SWGfL to see if I could connect via that and send an e-mail manually which it did with no problems. I cleared some non-delivery reports out from the queue and then all the e-mail seemed to flood out. Thought that was that and maybe an e-mail message had clogged things up although I've never seen Exchange do that before.
Checked it again late last night from home and once again e-mail was starting to build up in the queue; thats when I noticed the expired self signed certificate. I cloned the current one and deleted the old one but this then broke delivery from Outlook to Exchange (mail was stuck in peoples outboxes), but I had forgotten to enable this cert for SMTP so did that and then email was at least flowing from Outlook to Exchange but it was still not going out although external e-mail was coming in ok.
That's when I noticed this thread and decided to go to bed instead!
Come in this morning with a few teachers moaning at me about NDR's in the mailboxes. I made an annoucement in staff briefing to let people know there is a problem.
I've just changed the address of the SMTP Send Connector on the Hub Transport to MX.SWGFL.IFL.NET as per the suggestion here and e-mail has now rushed out and the queue is clear.
I'm now going to put a call into SWGfL to add weight to this issue.
Spoke to SWGfL directly earlier; it was a known issue but apparently down to a mass DNS update that occured. It would appear that our servers were still using the old IP address of the DNS entry. The 'mx.swgfl.ifl.net' is a relatively new address so that's why this is working but our standard smtp feed address should work. I've flushed DNS Caches but leave it on mx.swgfl.ifl.net for now and change it over later on.
This is what happened to me as well. Oh well nice of the SWGFL to inform me of this issue!
I hate exchange problems due to my lack of knowlage, not just with the software/database but really on how the whole email system works. I'll probably never upgrade to 2010 as I just know exchange will fail and so will it's backups! (as it hates me)