If Outlook or Windows Messaging reports No transport provider is available when trying to connect to VPOP3, it can mean several things. (This must be a contender for “the world's most meaningless error message”).
ERRORS.LOGfile in the VPOP3 directory to see if this gives an error message corresponding to the time you tried to connect using Outlook.
It may be that Outlook/Windows Messaging is contacting VPOP3 and VPOP3 is giving an error message. In that case, you should find the error message in the
ERRORS.LOG file, and you should look for more information on that error message.
It may be that Outlook/Windows Messaging is trying to contact VPOP3 but is unable to do so. This could be a TCP/IP addressing problem or a network problem.
It may be that Outlook/Windows Messaging is not even trying to contact VPOP3 but is deciding there is a problem before it gets that far. Outlook/Windows Messaging use MAPI 'Transport Providers' (this is what the error message means) to talk to mail servers, such as POP3/SMTP servers (like VPOP3), MS Mail or Exchange servers, or even other types of mail server. The 'No transport is available' error means that Outlook has gone through all the registered 'Transport Providers' and none of them has said that it can send mail to the email address you have specified.
A common reason that this can happen is if you have both the 'Internet Mail' and 'MS Mail' or 'Exchange Server' transport providers enabled (see the
Tools → Services option in Outlook. MS Mail and Exchange Server are both capable of handling Internet Mail addresses as well as the 'Internet Mail' service, but the 'MS Mail server' or Exchange Server may not be set up to allow Internet mail (eg if you are using the 'Workgroup Postoffice' which comes free with Windows). In this case, if the 'MS Mail' service has a higher priority than the 'Internet Mail' service, Outlook will never ask the 'Internet Mail' service if it can send the message, it will just ask the 'MS Mail' service which will initially say that it does support Internet email addresses, but later on say that it can't because the MS Mail server doesn't support it.
To get around this problem, set the order of the 'Services' so that the 'Internet Mail' service comes first, before either the 'MS Mail' or 'Exchange Server' services.