Are you using webmail (ie viewing via your web browser) or are you using an email client like Thunderbird or Outlook.
Using POP3 or Imap. ??
Lots of options but saving emails should not be overly difficult.
If using webmail, then unless your mailbox has capacity limitations, it’s highly lilely that they will remain in your PlusNet mailbox for as long as you leave them there.
Delete the ones you don’t want to retain, leave the others behind
Does the PlusNet mailbox have facility to create a seperate folder, i’d iamgine it does.
If so, create a new folder named ‘saved’, and move any emails from your inbox to the saved folder.
You may even be able to drag and drop from one folder to the other.
It should also be possible to create rules to do the same.
Something along the lines of:
If From Address = someone@somewhere.com
or if
Address = someone@somewhere-else.com
Then Move to ‘saved’ folder
Alternatively, if using an email client like thunderbird or outlook, perform the same.
Create a new folder within the client app, and drag & drop from one folder to the other.
If using POP3 on a client app (Thunderbird etc), when the emails are collected from PlusNet, it’s highly likely, that these will be removed at the PlusNet end leaving your mailbox empty.
If using Imap, they will only be removed at the PlusNet end when you delete them.
Anything on your client app, will undoubtedly be lost if your hard drive ever failed.
Anything on PlusNet, could potentially be there for ever or until you close your account.