Apple iOS Mail - Continued “Cannot Get Mail” Error and Sending Failure

I’m having continued issues accessing email via the Apple Mail app on both iPhone X (16.5.1) and iPad Pro (16.5.1) - I regularly get the “Cannot Get Mail - The connection to the server failed”/“Multiple Account Errors” messages on the iPhone, where one or more accounts can’t be accessed. It seems more often to affect Yahoo! Email accounts, and one in particular more often than others. The Mail app will often notify of a new email in an account which cannot be seen or opened; I can view and open these emails via a web browser without issue.

On the iPad, the issue seems to affect the same accounts, but sometimes the email subject is visible in the Inbox, but the email body is empty and showing “This Message Has Not Been Downloaded from the Server”.

I’ve removed and re-added all the accounts to the Mail app on both devices more than once, restarted etc, but it hasn’t fixed the problem.

I also have an issue on both iPhone and iPad where an email simply disappears when I try to send it - for example, I compose a reply to an email and click send, and am returned to the All Inboxes page. However, regularly the email does not send - it is not in the Sent folder or Drafts or any Outbox, and is lost. I’ve noticed that the Undo Send message does not appear when this happens, so presumably the problem happens before any attempt to send.

Has anyone resolved issues like these?