All if a sudden all our computers quit sending email. They still receive just fine. It's not our internet provider...we checked. And we can still send invoices via Quickbooks and files using YouSendIt...as they bypass our server. I'm beginning to think its our ISP but don't really know. I even tried installing a new email client and sending from that...no love. The outgoing mail goes into the outbox then "gets sent" and disappears from the outbox. But it never arrives. Since we live on email I need to get this fixed. Just hoping one of you brainiacs might have a fix.

Check sites like spamhaus and spamcop to see if your IP address has been blacklisted.

It can happen because of some overzealous anti spam falsely reporting you.

Easier still.... Type your IP address into dnsbl.info to check almost every blacklisting service worldwide in one go.

First I'll have to find out my IP address. But then I'll do just that.

Your ISP may have implemented port 25 filtering. I know att dsl does. Check with your email provider if you can use ssl ports or if they provide and alternate smtp port for sending mail.

I've heard that the postage rate has recently gone up, are you attaching the correct e-stamps? 
 
(All the really good answers were already taken.)

Well I did as Karyn suggested (Thanks, Karyn!!) and we were indeed listed on two of the scads of sites.  I've done ( I think) what's necessary to get removed but don't really know yet.  I'll keep ya posted.  According to those sites it sounds like a couple of weeks ago we were infected with a "bot" that was sending spam out through our address.  Go figure.  I hope this gets us working again and...to put a positive spin on it...at least I learned something from this.  Thanks again for that Karyn.
 

Now you can address craigb's (ITHRN) suggestion.

Yer most welcome hon.

Got delisted on both sites.  All looks good on dnsbl.info.  But still no outgoing email.  Does it usually take a day or two??