I once wrote that I would have to abandon Zimbra and my yahooo account.
Because I am now more or less dependent on it (lot of trouble to switch over again), I continued, but I would like to warn new searchers to NOT CHOOSE FOR ZIMBRA WITH YAHOO.

The software would be pretty good, if it were not for this problem:
SendMessage request failed: Human Verification required

This happens at random times; today I had it twice. I posted before
Will not send out mail??

and the solution (for one time) is here:
SendMessage request failed: Human Verification required

but it is VERY VERY irritating and frustrating when you are in a hurry and quickly want to send out a mail, and you encounter this and have to do a lot of work. If it would be for security- and anti-spam reasons, then I would suggest a comprehensive way, something like "after 10 more mails, you will have to human-verify your account." Not this bug that already hangs around a lot of time, and that the Zimbra or Yahoo-people seem totally unwilling to resolve or even explain.