I have my website hosted at one location, on a server that also accepts email, and I use Google Apps for mail as my email provider. I've had no problems in the last 18 months, and no reports of email being lost.
This week however, someone using Hotmail has tried emailing me. 25-50% of the emails they send are returned to them, with a "550 relay not permitted" message.
My MX records are correct, and checking the Exim logs on my web server I see that Hotmail is trying to send emails to the A record, because this server can accept email. This appears to be a known problem with Hotmail.
As I can't disable the webserver from accepting mail from Hotmail (since other people use the web server as their email hosting), is there a solution?