Assumptions can mislead, let's focus on the issue. Open an SSH terminal on your system and run this:
tail -f /var/log/exim/mainlog
Then try to send an email and tell us what you see. You can replace the email addresses in the log with a placeholder to keep that information private.
Please keep in mind that the only confirmed issue here on the forums is that /root/.forward requires a workaround. If we immediately aim to write things off as the result of a version number without checking details, we won't confirm any further issues because we're not doing due diligence.