I had been running ZCS 5.05 prior to my sending mail issue. The only change that occurred was my ISP (now Verizon FIOS). I can receive mail, but cannot send any mail - I receive the following error in the web client:

Could not send message: mail.SEND_FAILURE msg: SMTP server reported: null
code: mail.SEND_FAILURE
method: SendMsgRequest
detail: soap:Sender
data: [object Object]

The corresponding entry in the mailbox.log file is:
2009-01-06 16:05:28,134 WARN [btpool0-10] [name=XXXXX@XXXXX.XXX;mid=3;ip=204.52.215.14;ua=Zim braWebClient - FF3.0 (Win)/5.0.11_GA_2695.RHEL5;] smtp - Exception occurred during SendMsg:
com.zimbra.cs.mailbox.MailSender$SafeSendFailedExc eption
at com.sun.mail.smtp.SMTPTransport.issueSendCommand(S MTPTransport.java:1502)
at com.sun.mail.smtp.SMTPTransport.mailFrom(SMTPTrans port.java:1041)
at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTr ansport.java:621)
at javax.mail.Transport.send0(Transport.java:169)
at javax.mail.Transport.send(Transport.java:98)
at com.zimbra.cs.mailbox.MailSender.sendMessage(MailS ender.java:535)
at com.zimbra.cs.mailbox.MailSender.sendMimeMessage(M ailSender.java:253)
at com.zimbra.cs.mailbox.MailSender.sendMimeMessage(M ailSender.java:118)
at com.zimbra.cs.service.mail.SendMsg.doSendMessage(S endMsg.java:189)
at com.zimbra.cs.service.mail.SendMsg.handle(SendMsg. java:154)
at com.zimbra.soap.SoapEngine.dispatchRequest(SoapEng ine.java:433)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.jav a:288)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.jav a:167)
at com.zimbra.soap.SoapServlet.doPost(SoapServlet.jav a:269)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:727)
at com.zimbra.cs.servlet.ZimbraServlet.service(Zimbra Servlet.java:189)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(Ser vletHolder.java:487)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1093)
at org.mortbay.servlet.UserAgentFilter.doFilter(UserA gentFilter.java:81)
at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter .java:132)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1084)
at org.mortbay.jetty.servlet.ServletHandler.handle(Se rvletHandler.java:360)
at org.mortbay.jetty.security.SecurityHandler.handle( SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(Se ssionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(Co ntextHandler.java:716)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebA ppContext.java:406)
at org.mortbay.jetty.handler.ContextHandlerCollection .handle(ContextHandlerCollection.java:211)
at org.mortbay.jetty.handler.HandlerCollection.handle (HandlerCollection.java:114)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:139)
at org.mortbay.jetty.handler.rewrite.RewriteHandler.h andle(RewriteHandler.java:350)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:139)
at org.mortbay.jetty.Server.handle(Server.java:313)
at org.mortbay.jetty.HttpConnection.handleRequest(Htt pConnection.java:506)
at org.mortbay.jetty.HttpConnection$RequestHandler.co ntent(HttpConnection.java:844)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser. java:644)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpPa rser.java:211)
at org.mortbay.jetty.HttpConnection.handle(HttpConnec tion.java:381)
at org.mortbay.io.nio.SelectChannelEndPoint.run(Selec tChannelEndPoint.java:396)
at org.mortbay.thread.BoundedThreadPool$PoolThread.ru n(BoundedThreadPool.java:442)
2009-01-06 16:05:28,134 INFO [btpool0-10] [name=XXXXX@XXXXX.XXX;mid=3;ip=204.52.215.14;ua=Zim braWebClient - FF3.0 (Win)/5.0.11_GA_2695.RHEL5;] SoapEngine - handler exception
com.zimbra.cs.mailbox.MailServiceException: SMTP server reported: null
ExceptionId:btpool0-10:1231275928134:4ec07debec60e32e
Code:mail.SEND_FAILURE Arg: (unsent, STR, "My Name <XXXXX@XXXXX.XXX>")
at com.zimbra.cs.mailbox.MailServiceException.interna l_SEND_FAILURE(MailServiceException.java:441)
at com.zimbra.cs.mailbox.MailServiceException.SEND_FA ILURE(MailServiceException.java:453)
at com.zimbra.cs.mailbox.MailSender.sendMimeMessage(M ailSender.java:323)
at com.zimbra.cs.mailbox.MailSender.sendMimeMessage(M ailSender.java:118)
at com.zimbra.cs.service.mail.SendMsg.doSendMessage(S endMsg.java:189)
at com.zimbra.cs.service.mail.SendMsg.handle(SendMsg. java:154)
at com.zimbra.soap.SoapEngine.dispatchRequest(SoapEng ine.java:433)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.jav a:288)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.jav a:167)
at com.zimbra.soap.SoapServlet.doPost(SoapServlet.jav a:269)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:727)
at com.zimbra.cs.servlet.ZimbraServlet.service(Zimbra Servlet.java:189)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(Ser vletHolder.java:487)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1093)
at org.mortbay.servlet.UserAgentFilter.doFilter(UserA gentFilter.java:81)
at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter .java:132)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1084)
at org.mortbay.jetty.servlet.ServletHandler.handle(Se rvletHandler.java:360)
at org.mortbay.jetty.security.SecurityHandler.handle( SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(Se ssionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(Co ntextHandler.java:716)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebA ppContext.java:406)
at org.mortbay.jetty.handler.ContextHandlerCollection .handle(ContextHandlerCollection.java:211)
at org.mortbay.jetty.handler.HandlerCollection.handle (HandlerCollection.java:114)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:139)
at org.mortbay.jetty.handler.rewrite.RewriteHandler.h andle(RewriteHandler.java:350)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:139)
at org.mortbay.jetty.Server.handle(Server.java:313)
at org.mortbay.jetty.HttpConnection.handleRequest(Htt pConnection.java:506)
at org.mortbay.jetty.HttpConnection$RequestHandler.co ntent(HttpConnection.java:844)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser. java:644)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpPa rser.java:211)
at org.mortbay.jetty.HttpConnection.handle(HttpConnec tion.java:381)
at org.mortbay.io.nio.SelectChannelEndPoint.run(Selec tChannelEndPoint.java:396)
at org.mortbay.thread.BoundedThreadPool$PoolThread.ru n(BoundedThreadPool.java:442)
Caused by: com.zimbra.cs.mailbox.MailSender$SafeSendFailedExc eption
at com.sun.mail.smtp.SMTPTransport.issueSendCommand(S MTPTransport.java:1502)
at com.sun.mail.smtp.SMTPTransport.mailFrom(SMTPTrans port.java:1041)
at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTr ansport.java:621)
at javax.mail.Transport.send0(Transport.java:169)
at javax.mail.Transport.send(Transport.java:98)
at com.zimbra.cs.mailbox.MailSender.sendMessage(MailS ender.java:535)
at com.zimbra.cs.mailbox.MailSender.sendMimeMessage(M ailSender.java:253)
... 33 more


I've been working on this for the past two weeks and I don't know what else to try - any help is greatly appreciated!

Thanks,
Matt