It's been for 3-4 days that my site doesn't send any emails to users. Prior to this issue, neither me nor my hosting make any changes in my site's code and the hosting.
It should be mentioned that my other non-Q2A sites on the host send emails properly.
How can I solve this problem?
You can try upgrading to Q2A 1.7.3 as that has a more recent version of the PHPMailer component. Or you could manually upgrade just PHPMailer - download the latest version here, then copy the appropriate files to the qa-include/vendor/PHPMailer directory.
Besides that, if mailing has stopped working without you changing anything on the site, then it must be some kind of problem with the mail server. Make sure you have all the details correct and that you're not going over any sending limits and so on.
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May 7, 2020: Q2A 1.8.4 released!