Just some info… I've been running previous versions of iRedmail for the last year, but have had trouble lately with the new release. I was getting "unable to resolve local hostname" errors with very little other debug info. After loading both the Ubuntu and RHEL versions, nothing seemed to help. Then someone posted that they had similar problems but the squarecube client worked perfectly, and I realized the errors were entirely due to the Thunderbird client. So the problem wasn't the new iRedMail version, it appears that it is the new Thunderbird client version, and it seems unable to cope with invalid/non standard config settings on a mail server. My problem turned out to be a site with an invalid mx record that didn't cause problems before but now gets stuck between iRedmail and Thunderbird. I've swapped my email client and  no longer have any issues (roundcube works perfectly as does outlook).
It never occurred to me that it could be a client issue or an issue outside of the iRedMail installation, so hopefully this will save others from wasting time, looking for a server fault. I will also pay more attention to how other mail servers are configured, without assuming they are correct.
Thanks for a great mail suite, and for the amazing amount of online documentation and support.   
Amazing site....