All useful stuff for sure. Is Authenticated SMTP needed for IMAP? My provider likes to charge a premium for this, which irks. Likewise, they offer DMARC, at no cost, but also with no laymans explanation that I can understand, save to think that I don't need it.
IMAP is for your inbound mail.
SMTP is for mail you send, outbound.
They are not really directly related.
SMTP is historically not authenticated.
But this has become a problem, allowing spammers to relay mail through unauthenticated outbound servers.
So these days, ISPs will only allow unauthenticated SMTP (outbound) mail from their customers, on their internal IP addresses.
If you try to connect from outside your ISP, eg from a phone on a 4G network, they will reject it.
To allow for this, you need to enable SMTP authentication.
Then, your phone can send mail whilst connected to the Internet from external IP addresses.
And DMARC: Just enable it if it's a check-box.
It's beyond the scope of this post to say what it does, but if it's not enabled then big mail providers like gmail will reject your mail.