As the issue is impacting a number of people there's a team working on the problem. What email domain are you using? (e.g. talktalk, lineone, tiscali)
Service status is reporting a fault and there will be updates:
"We're aware of some customers having issues when they try sending, replying or forwarding emails. Our engineers are looking into this as a priority, and looking into fixing as soon as possible. We apologise for any inconvenience caused."
In the meantime try using an email client like Outlook or Thunderbird or an email app like gMail.
Hi mike1232, I'm told by the Email Admins that they've cleared this fault. Can you post back to let me know if it's working normally now?