Metronet emails - repeated password requests -and "on behalf of"

[#1]

I am on Outlook and Metronet  for emails - I have the same problems with being repeatedly asked for username and password - which, having learned from another thread, I appeared to cure by adding @Metronet.co.uk to my user name in outlook - tools - change.

But now when I send an email the recipient sees not my metronet account but one of my plusnet accounts with "on behalf of XXX@metronet.co.uK"

Thi is very confusing for recipients of my emails

This morning the repeated requests for name and password returned.

Does Plusnet have this in hand?

[#2]

Over the past day or so I've been getting various SMTP error messages, mainly 
-  invalid username/password
-  too many recipients in the past hour
-  server is refusing connections
....plus (I think) others that I haven't recorded. 
No apparent pattern to these. 
Initially these problems appeared to have been fixed (via the advised "add @metronet.co.uk" change),  and things were working inc. outgoing mail,  and appeared stable for a couple of hours yesterday,  but that didn't last....

[#3]

UPDATE: SSL/TLS is NOT available for Metronet users.

Encryption should be set to NONE.  That will eliminate the certificate issues.