My wife using an iPhone on IOS 15.1 cannot send an email to a specific gmail account, getting the error message ‘connections to outgoing server mail.btinternet.com failed. It has sat in the outbox for two days.
However, she can send emails to many other recipients, including to me on my gmail address as well as my btinternet address.
She sent to problem email text to me from her iPhone to my iPad using my gmail address and I created a new email to her original addressee , using her email account on the iPad. This also failed with the same message.
This is not the first time this has happened. Previous problems seemed to disappear eventually without ever understanding the root cause.