MX Settings - help please

[#1]

Hi
I'm new to Plusnet Business.
First the background.  Our .com and .co.uk domains and email were provided by register.com in the US.
Having moved into offices and as we're using Plusnet for telephone and broadband I thought it would make sense and be cheaper to move everything to Plusnet.
So I followed the instructions for transferring the domains with full control to Plusnet.
There have been shome hiccups but I think .co.uk has now transferred as a whois lookup suggests so and shows the renewal date to have increased by two years (both domain registrations having been due to expire today/tomorrow).
The .com domain seems to have gone through too but that is less urgent/of an issue as we don't use it for anything, just forward traffic to it to .co.uk.
Our website is hosted by a third party.  The DNS record has successfully been added with the Plusnet domain managment interface.
However I am having email problems.  I set up mailboxes with plusnet which all say domainname.plus.net but was told this is ok.
We have been getting all emails except if I sent emails from my personal email (also with register.com) it would not get through or bounce back.
So as Plusnet are slow getting back I had a look at my Register.com account today and found various setttings still set for .co.uk and also our email accounts with them still active but I couldn't access their webmail so I decided to delete everything.
I then tried sending an email from my personal address to the work .co.uk again and this time I received a failure notice saying the mailbox doesn't exist.
I used the tool on mxtoolbox.com to see what was showing for .co and .co.uk and found mx.avasin.plus.net in respect of .com and I forget in respect of .co.uk
When I looked at the additional DNS records section within Plusnet domain managment for .co.uk there is an A record setup pointing to where the website is hosted.  There is a cname record set up with the left field being www and there was no MX record showing.
Should there be an MX record and if so, what?  Bear in mind I want to use Plusnet's mail servers and our domain is registered and hosted with them too.  I did try swapping it to mx.avasin.plus.net but I'm just guessing.
I really need this to be sorted asap as we could lose important business emails.
Just to add that we want POP3 mail and that is what has been selected.
Thanks
Dan

[#2]

Hi Dan,
You've probably already sorted this by now, but just in case:

Quote from: Dan1502

When I looked at the additional DNS records section within Plusnet domain managment for .co.uk there is an A record setup pointing to where the website is hosted.  There is a cname record set up with the left field being www and there was no MX record showing. Should there be an MX record and if so, what?  Bear in mind I want to use Plusnet's mail servers and our domain is registered and hosted with them too.  I did try swapping it to mx.avasin.plus.net but I'm just guessing.

If the domain is configured to receive email using POP3 then no, you won't see the MX record listed under the additional DNS records section. That's not to say it isn't there though.
You'll get a definitive answer by performing a lookup against the domain in question e.g. from a Windows command line:

nslookup -type=mx domain.tld