What's the email domain? i.e. talktalk.net or tiscali.co.uk or another?
Is the email address the same as you've got registered for this Community profile? If it's different put a note of the email address in Private Notes which is at the bottom of the page of your Community Profile Personal Information (Click here). Add and Save changes.
It's likely that it's just a mailbox repair job that needs running.
As the issue is impacting a number of people there's a team working on the problem.
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."
I am having a similar problem. Extraordinarily frustrating. Starting on 26 March, if I Reply, Forward or Send from Draft on Talktalk webmail, the dialogue box comes up and says "Sent", but it was not sent and does not appear in Sent Items. If I send the same email form Compose, it works perfectly. This error repeats on Safari, Edge and Windowsphone platforms.
Very very crap from talktalk!