iphone 6 email server issue

[#1]

I have followed Plusnet guidelines for setting up emails on my iphone 6s but only the incoming emails work. I am unable to send emails, despite being able to via relay.plus.net on my desktop & previously on my Blackberry.  Error message is "The username or password is incorrect". However, Plusnet Support has confirmed I have the correct User name and password. Has anyone got any suggestions please?

[#2]

Does the email account belong to a free or paid-for account? If it's the former then you won't be able to use SMTP authentication and will need to be connected to the Plusnet network in order to send emails using relay.plus.net.