I've had similar problems. While I can usually send and receive mail via Outlook without trouble, sometimes it will receive but not send. When I try to go into my webmail to reply from there, it tells me that I have to log in in order to use it, although I am using the correct email/username and password, and am able to access my mailbox settings.
Then, when I contact tech support, they tell me that they aren't having any trouble accessing my webmail.
A few hours later, everything starts working again, but only to reoccur periodically.