FYI - Strange issue with UPC SMTP Server

I have been using UPC broadband for over a year without any major issues.
I use their SMTP server at smtp.upcmail.ie
Over recent months I have been getting random refusals to send emails.
I get the message "550 5.1.1 Relaying denied to domain randomdomain.whatever"
Normally this means you are not a customer of the ISP.
After wasting an hour or more talking to 'tech support' I finally got through to a second level person who was excellent.
He explained to me that their outgoing server required authentication - i.e. my UPC email address and my UPC email password. I was unaware of this.
BUT, and this is the strange thing, it only does very infrequent random checks of the authentication!! He told me you could go for months without a check.
Of course they always do the normal check that you are coming from a UPC router i.e. you are a UPC customer.
So, why the random authentication checks? A mystery!!
Or can anyone explain the thinking behind it?
G.