Sending emails from non-Plusnet addresses

[#1]

In the domain, which has yahoo.com as the default, enter your domain and select TXT from the pop up list in Type then hit go.
Most domains do not have spf records but below is an example for itunes. Then if the receiving server is checking the spf it does a DNS lookup to get the IP address of the servers that are allowed to deliver emails for the domain. With the example below for itunes the IP range of the servers are 17.0.0.0/8 (which give an IP range of 17.0.0.1 - 17.255.255.254). Checking the header of a received email from itunes gives Received: from badger03003.apple.com ([17.254.6.126]) by and 17.254.6.126 is in the range specified by the spf record. 
For your domain you are looking for a txt string that contains =spf1.

[#2]

Thanks, itsme.  I've tried this and didn't see any references to spf.  I have therefore passed the ticket back to Plusnet, asking them to investigate further.

[#3]

I would be surprised if this is down to PN servers as these are relay smtp servers and normally they do not do any checking. So long as you have connected to them by a PN IP or logged on to them.
Are the failure seen from different recipient addresses of just the one?

[#4]

Every address I've tried fails - it makes no difference who the recipient is.  I can send to any of the addresses via my Plusnet email address, but as soon as I use one of the addresses from my domains it fails.  If the external host from which I get my domains hasn't implemented spf, is there anything else they could have done to create a block?  It's bizarre how it was working fine on Friday and then the error started occurring on Saturday morning.

[#5]

Did a search on the error code and found a single reference that it could be caused by anti-virus programs. Have you installed or updated AV programs on Saturday?

[#6]

I thought it could be to do with the mail scanner in my AV package, so I disabled that, and also the firewall, but I still had the problem (and, in any case, on Saturday morning when I first tried, the AV software hadn't been updated since Friday lunchtime).  To be absolutely sure, I have also tried setting up my mail on a computer with no AV installed - and the same problem occurred.

[#7]

Just as an aside, my problem was fixed by ticking the authentication box in the server settings. The hosting company forgot to mention that change when they migrated my account to a different mail server.

[#8]

Glad to hear you got your mail issue sorted, geewizz.
I've passed the support ticket back to Plusnet, and have received a message this morning advising me that it has been allocated a fault number and is currently under investigation.  Watch this space...!

[#9]

@garywood84
I have just tested this on my own personal Plusnet account, i successfully managed to send an email using an externally hosted domain email address and the Plusnet relay servers, relay.plus.net to a personal gmail account. I then decided to try using one of the email address's you specified to see if this would go through fine using the Plusnet relay servers, and this again successfully went through without any error messages.
This would therefore lead me to believe that either your local config is incorrect some how, or the recipients you are trying to send to are simply not working.
Can you please confirm that you are using relay.plus.net as the outgoing relay server, you are connected on the plusnet platform, and that you are using no form of authentication against the relay servers when trying to send an email?
If all the above works, could you please try and setup a new account in outlook using the email settings found on the website support section.
Regards,

[#10]

Dan,
Thanks for your message.  I can confirm that I was using relay.plus.net as the outgoing server, that I was connected via Plusnet and I wasn't using any form of authentication last week.  It was fine on Friday, I didn't change anything, and it just stopped working over night.  On Sunday, when it still wasn't working, I changed the SMTP server to allow me to send mail, and have been using that ever since.  I created a duplicate copy of my mail account to test with the Plusnet server again yesterday, and it was still rejecting my messages.  However, I've tried it again today, and it's working again!!  The settings are definitely exactly as they were at the weekend - I've checked the account in my backup from Friday afternoon.
This is obviously really bizarre.  The problem can't be on my side, since I've not changed anything and it's now just working again, so either something has happened at Plusnet or my external host to make this work again.  Anyway, for now at least, it's fixed! 
Thanks again to everyone who has posted in this thread to help me get this resolved.
Gary