We have Thunderbird on all our machines; three different operating systems, but a majority are Linux Mint. Startup on Linux is 3s to 6s depending on the machine

We have Thunderbird on all our machines; three different operating systems, but a majority are Linux Mint. Startup on Linux is 3s to 6s depending on the machine. All machines have mulitple accounts, from two to six, some use IMAP4 and some use POP3. At various times we've tested other clients, but Thunderbird is the only one that has all the features we want. These include complex filtering, calendaring (external and our own iCal server) and productivity add-ons. At the end of every year we archive all mail that's more than a year old into a new Thunderbird profile, so there's now a list of historic profiles for some accounts going back twelve years. That does make searches for specific messages a hassle, but these are thankfully rare, and it means the current profile is relatively trivial for backups and history is available to all machines via our local server.