Steps to reproduce:
My hostname is mx1.lewisjenkins.uk
1) Add a virtual server called mx1.lewisjenkins.uk with 'Apache Website' and 'Mail for Domain' enabled.
2) Edit the virtual server, remove 'Mail for Domain'.
THIS HAPPENS (from Webmin log) :
Changed file /etc/postfix/main.cf 35c35 < mydestination = mx1.lewisjenkins.uk, localhost.lewisjenkins.uk, , localhost --- > mydestination = localhost.lewisjenkins.uk, localhost
3) Delete Virtual Server 'mx1.lewisjenkins.uk'
POSTFIX IS NOW BROKEN, REFUSING TO ACCEPT MAIL.
It took me a few minutes to figure out what had happened. Manually adding mx1.lewisjenkins.uk back onto mydestination and restarting Postfix fixed it, but it was a scary few minutes.