Get our superfast unlimited fibre broadband
Great-value deals on unlimited broadband
Flexible and affordable sim only deals with no contract
Our network reaches over 99% of the UK, so we've got you covered
Low on minutes, data or texts? Just add one of our bolt-ons
Quick and easy access to your account, services, bills and tools
View and manage your mobile account, tariff and usage
Go to the fix.
It sounds to me a classic case of lack of SMTP authentication.
On the iPhone
> Mail, Contacts, Calendars
Select the account under "account"
> Outgoing Mail Server
and select "password"
If you can't change, it add a new SMTP server which is authenticated and turn the other one off for the account.