The SMTP class extends the mailhandler class and defines two new variables: "$data" and "$code". However the mailhandler class accesses those variables in "fatal_error".
Hi,
Thank you for your report. We have pushed this issue to our Github repository for further analysis where you can track our commits and progress with fixing this bug. Discussions regarding this bug may also take place there too.
Follow this link to visit the issue on Github:
https://github.com/mybb/mybb/issues/2101
Thanks for contributing to MyBB!
Regards,
The MyBB Group