Update MailPanel.php fixes #507

[squio]

Fix for issue #507

Explicitly convert headers to string

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌
Fixed issues	507

[what-the-diff[bot]]

PR Summary

• Header conversion improvement The headers are now converted to a string format to enhance compatibility and ensure proper functionality.

[samdark]

Thanks.