@Paul_Taylor I'd be willing to bet his system is being abused and he doesn't know it. I went through a period where we were often blocked by larger carriers, and it turned out it was happening because spammers had broke accounts by brute force then used them to spam. Adding fail2ban to stop the brute forcing combined with a filter that looked for e-mail on a single account coming from three or more nations on a 24 hour period and if that happened locked the account and forced a password change solved the issue.