iOS Email Configuration Settings

iPhone 6, iPad Mini, iOS 10.3.2:

Running private domain POP3 mailboxes on 1and1 host servers

Recently found my outgoing server blocking my email addresses so called tech support.
Confirmed all accounts names, passwords, ports, etc.
Saw iOS said username and password optional on outgoing (SMTP) server but tech said to add it.
iOS verified account and email send started working.

Went back to review configuration settings and found outgoing server OFF on iPhone and without specified username and password entries listed on iPad.
Both devices now sending email OK...

Tech said iOS automatically configures outgoing server on phone and differently on iPad once credentials are entered, but differently on each device.
What?
This all looks so different from any Apple support documentation on the devices I am just blown away.
How can the phone send without the SMTP server on?
Why does the iPad take the username and password, verify the account, then later not display those entries?

I am happy the have my SEND function back but totally confused about the configuration settings issue.

Any thoughts?
Thanks,
Ron

--- Post Merged, Jun 29, 2017 ---

So I have now configured the other 3 email accounts specifying username and password for the SMTP server and automagically the outgoing server is now ON!
And the accounts all work...
Trying to pull up an Apple reference document on setting up email accounts for iOS 10.3.2
Any links out there?

 

Final point...
I originally set up my iPhone configuration settings 3 years ago and it worked first time...
I recently moved my domain to a new host and modified the settings to reference their incoming and outgoing servers and it worked fine for a month.
Then, out of the blue 2 weeks ago my iPhone emails started failing, incoming and outgoing.
That just makes no sense to me.
Anyone else ever have this kind of problem?