Well it means that Tesco think that Bravenet is not a legitimate source of emails for your domain.
This could be down to actual spam reports - people are lazy and hitting the "report as spam" button is a lot easier than going through the proper unsubscribe process. This is not necessarily down to your mailing list, it could be any that is run through Bravenet.
It may be algorithmic - from a whois query your site appears to be hosted by UK2 in London but the emails are coming from Canada.
I gave up on mailing list services many years ago as there are too many complications like this that I can't control and maintain my own list manually (a fairly small B2B list). I am a UK2 customer as well and have no problem mailing from their servers.
Tesco may act on an approach direct from Bravenet, you will need to raise a support ticket to start the process.