I have an iPhone and MacBook Pro both with latest OS's at 07 April 2020. I recently have taken on a superfast hub which, after a few weeks has decided to make my email life hell by not selecting incoming emails from the server, also appearing to send emails which then do not show in 'sent' but turn up in an Outbox.
Repeated attempts with failed outbox captures may show an apparent departure with the same result above. What also tends to happen (not all) is most initial and later attempts to send a given email turn up as sent and arrive at the intended recipient up to hours later.
On occasion, a failure to reach the outgoing server message pops up.
I have tried airplane mode, restarting the hub, toggling VPNs with varying success but the hub now seems to have created its persona non grata list. But I have now stumbled on a continuing success method for a whole week: Swap the superfast for BT Wi-fi With FON. Maybe a pointer to what's wrong?