Zimbra unusable; many times "Human Verification Required"

[tamanes2]

After using Zimbra Desktop successfully during a month or so, suddenly I got this error
when trying to send an email:
Delivery failed: SendMessage request failed: Human Verification required

With the necessary pain I found the solution, see
SendMessage request failed: Human Verification required
and I could use it another few weeks.
But now the problem reappears more and more frequently, and now every few days, and it makes the program
completely useless. One cannot apply this solution every time he wants to quickly send a mail.
Unfortunately I´ll have to abandon it.