Error when sending form

Hi.
After upgrading to Contact Form 7 V 6.0 I receive a lot of mail_failed errors when people are sending messages (6 of 10 sent messages have mail_failed errors so far).
The message appear in the Flamingo list over incoming messages, but the plugin won’t send the actual e-mail to me. I tried to add different SMTP plugins to see it that would do the trick – if it was a problem with WordPress mail feature, but the error keeps on coming. What I can see here in the forum others have the same issue, so it looks like something is not working properly with the plugin for sending messages as this has been more or less flawless before 6.0.
Thank you.

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