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.
Thanks for updating your Community Profile Personal Information (Click here) to include the email address affected in Private Notes box at the bottom of the page. Save changes. The TalkTalk Community support OCE will then be able to check your email account. The Easter holiday means that this follow up will not be until Wednesday onwards. But this topic is in the workflow so you will get a response.