I changed zimbra to run on port 81 using:
zmprov ms zimbra.mydom.com zimbraMailPort 81
zmprov ms zimbra.mydom.com zimbraMtaAuthHost zimbra.mydom.com
Users can login, send and receive mails etc. However when I login as admin and attempt to view somebody's mail - by going to accounts, highlighting user then clicking 'view mail' I get an error page.

View mail attemps to open a new browser tab pointed at:

which times out. If I manually remove the :81 from that url it works fine.

Any ideas what to do? This function is important in our use case.