Good to know that the important emails are being received by your iCloud email account.
What's the sending email domain? I can make a first step at looking at the mail servers in use.
I may need you to look at the emails received in iCloud and view the raw header (View source or view header) and tell me what the email domain is in the <chevron brackets> on the Return-Path: line. That domain is the one checked by the inbound TalkTalk Mail servers.
Have you asked the originating company to ask their email admins to check that your TalkTalk Mail address is not added to a mail suppression list? This is a standard practice when there's a bounceback from TalkTalk indicating a potential problem with delivery. However, the sending mail service may not discriminate between a temporary delivery fail and a permanent delivery fail and as a result will stop sending mail regardless of your mailbox being active.
