Are you using webmail on any device or are you viewing mail messages on email apps on both devices?
Your description suggests that one or both devices are set up with POP3 email accounts on email apps. POP3's default setting downloads and deletes mail from the online Inbox. So if one device does that then the other device cannot download the message because it's been downloaded and deleted and exists only on the first device.
If the second device is also set up as a POP3 account using default settings and that downloads and deletes mail from the online Inbox then that mail message exists only on the second device.
Either set up both devices to never delete from the server / save a copy on the server OR save your wanted mail messages, delete both account setups and reinstate as IMAP email accounts.
As a former TalkTalk customer are you subscribed to or planning to subscribe to TalkTalk MailPlus to retain the full use of your email addresses? TalkTalk will usually ask if you wish to subscribe after you leave TalkTalk if your tiscali.co.uk address(es) was/were previously added to the ceased customer MyAccount.
The TalkTalk Mail Support Hub is your go to resource for information, guides and Community support for TalkTalk Consumer home broadband and MailPlus subscribers.
Select here:
TalkTalk Mail Support
TalkTalk Mail help
Using TalkTalk Mail
Changing your email password
Email settings and device setup guides for TalkTalk Mail
About TalkTalk Mail Plus
Subscribe to TalkTalk MailPlus
Cancel TalkTalk MailPlus
