Mac OS 'El Capitan' - Email settings?

[#1]

I have just installed 'El Capitan' and have examined all available help, but without Plusnet's specific requirements, none of these work. Plusnet's own email settings help does not include 'El Capitan' - maybe they haven't heard of it yet in Yorkshire!
Is there anywhere in the Plusnet shed at bottom 'at garden where there's ought might help?

AFTER ABOUT 8 HRS I HAVE MANAGED TO CONNECT WITH 'MAIL' NOT SURE WHY JUST PERMUTATIONS, ELIMINATION AND COMBINATIONS.
HINT - AFTER EVERY CHANGE/NEW ATTEMPT - QUIT MAIL AND OPEN UP AGAIN. ANOTHER PROCEDURE TO ADD!

[#2]

Hi,
just had a similar difficulty which Plusnet were unable to solve. They have no support notes for el capitan.  I have had to check the box "allow insecure authentication" in accounts  before I could access my mail. Thanks for the info re quitting and restarting Mail. Crazy.
Any idea what this means? It doesn't sound like the kind of box I really want to have ticked.
Thanks
Ollie

[#3]

Does seem bug ridden this Apple software... 
I have seen no end of issues with my iPhone - sometimes (for no good reason) it has been necessary to trash the email account and start again.
Setting up email should be simple unless the email client has made it difficult as did the first release of Win10.  People were unable to specify the user account name, due to the client software making gross assumptions about the email account configuration (user account name being the same as the email address).  Not so long ago an Apple upgrade presumed that SMTP authentication and SSL were obligator - these are not decisions for client software producers to make.  It is not PlusNet's responsibility to support every deviant software product, rather it is the responsibility of the software vendor.
PlusNet server expect a user name - your PlusNet account name - in the case of a sub-mailbox this is specified as username+mailboxname
Some people have successfully logged into their PlusNet email services using the email address, but it is not consistent across all accounts - you can see if this works for you using the webmail interface.

[#4]

Well you are quite right in your observations re the mega apple monster. However, plusnet make a load of dosh out of apple users so I would expect them to firstly be aware of and secondly to support every apple operating system as soon as it has been released. They could also send out an email to people like me saying that they will not support a particular OS and avoid this kind of problem.
But what do I know. 

[#5]

Whilst I do understand your frustration, I think your expectation's are unrealistic.  This is not a matter of each and every ISP supporting each and every change in OS (no matter who it comes from), rather it is a matter of OS providers offering products which function to the established standards and principles.
The presumption that the user account is the same as the email address is a seriously inept assumption on the part of any email client provider.
The attempt by some software vendors to 'dumb down' the user interface has caused no end of issues for non-tech-savvy users.  Indeed even some tech-savvy users have run into great difficulty.  Recently I migrated a number of email users from one MS Exchange server to another.  During the process I had a need to reconnect to the old servers.  No matter how I tried, the dumbed-down interface on MS Outlook would not permit manual configuration of the old server details.  It insisted on using auto-configuration and thereby could not connect to the old servers.
Ironically (given the context of this thread) the Apple platforms simply offered full manual configuration when auto-discover failed.  The issues are with the mail clients, not the mail server providers.