Email forwarding help, so confused!

[#1]

Please can anyone help, tearing my hair out here.
I have recently moved my business domain from 1and1 to 123reg.  Plusnet are my ISP.
I have lost all business emails and have been trying unsucessfully to set up the forwarding email addresses that were working fine with 1and1 but i just get errors.
I am really confused by the alias and mailboxes in plusnet. 
I had a  mail@mybusinessnamehere.com email address and my partner had a  hisname@mybusinessnamehere.com
However I try and set up aliases, redirects or mailboxes, I just cannot get either of them to work any more.
I have tried all the step by step guides on 123 reg and plusnet, but something is going wrong.
Partner using windows mail and I am using windows live mail.

[#2]

The first question is whether the emails work on 123reg - I assume that you have set up the emails with them and have checked via their web mail system that you can send and receive
The basic thing is that you can only receive emails using the Plusnet servers where the domain is hosted on Plusnet or where the email address is the Plusnet one
If the domain is hosted on Plusnet that isn't a problem but where it is hosted elsewhere you would need to use that hosts email server to receive emails via your mail program
This should explain how to do it http://www.123-reg.co.uk/support/Downloadable/Downloadable-Guides/my-email-guide-1429/

[#3]

Quote

The basic thing is that you can only receive emails using the Plusnet servers where the domain is hosted on Plusnet or where the email address is the Plusnet one If the domain is hosted on Plusnet that isn't a problem but where it is hosted elsewhere you would need to use that hosts email server to receive emails via your mail program

Technically correct, BUT the basic ( and cheap ) domain hosting packages often don't provide facilities to store and access email. Instead they provide an email forwarding facility whereby their server accepts the incoming mail but then just forwards it to where you specify. Obviously you need a valid email address to forward to.
If that's the situation in this case then PlusNet mailboxes need to be created for the forwarded mail.
For the example above you would create mailboxes somename@username.plus.com and someothername@username.plus.com  ( where username is your PlusNet user name ). Then on the 123 reg control panel you forward mail@mybusinessnamehere.com to somename@ username.plus.com and hisname@mybusinessname.com to someothername@username.plus.com. You should then be able to collect mail with your mail client from the 2 plusnet mailboxes. Make sure that you set the email address  in the mail client  to the @mybusinessname.com address so that outgoing mail appears to come from your domain and then manually configure the server settings to specify username+mailboxname as the logon user name ( see guide here http://www.plus.net/support/email/setup/windows_live_mail_2011_setup.shtml ).
As to alias's, these are simply a method of assigning additional name(s) to an existing mailbox such that the the PlusNet servers will accept mail to somealias@username.plus.com and direct it to the mailbox to which the alias is assigned.
Hope that makes sense and post back with any queries

[#4]

I'm certainly using Plusnet with 123 email forwarding without problems - either with Outlook Express or webmail.
I would say however that the 123 Control Panel isn't the easiest interface to use, sometimes has problems, the help screens are out-of-date and there appears to be no way of ringing them up to discuss. I have to raise a question through the website when there is a problem and it's difficult at times to get them to understand what the problem is. For instance catch-all forwarding was a problem several months ago and it took some time to get it sorted.
I can however say that even 123 are easier to deal with than 1and1 which I found a nightmare to liaise with.