Question Could not instantiate mail function

I have the same problem again as (see my topic 4 weeks ago) but now the problem occurs again.

I did update my site to the latest Joomla version and Kunena version. Testmailing from the site works. Both when using phpmail or smtp mail.
When I use php mail I get an error when a user posts something on the forum : Could not instantiate mail function
When I use smtp mail I get the following error : The following From address failed: <mailadres> : MAIL FROM command failed,,,SMTP server error: MAIL FROM command failedSMTP server error: MAIL FROM command failed

Any ideas how to find the root cause of this problem?

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I did disable HTML mail and the plain text mails are working.

What could cause this kind of problems? 

Hello,

Which SMTP server are-you using ?