As a further update, weirdly, I can send this link to my Virgin Media account, and it'll turn up in my inbox just fine. But I can't send it from it, because it thinks it's spam.
Any advice would be hugely appreciated. The full error message is:
An error occurred while sending mail. The mail server responded: 5.7.0 SPAM Content Found (VM603) crwLmi5chI8uBcrwLmJ5NB. Please check the message and try again.
The email is literally just my signature, although I have also tried writing more just in case that's what's triggering the filter. Seemingly, the thing that's causing the problem is the link to my website.