Bug Summary
Mailhog has a reported bug Mail subject displayed as empty when encoded · Issue #282 · mailhog/MailHog · GitHub which means that longer subject lines are empty
Steps to reproduce
Send an email with a subject line over 47 characters
Read email in Mailhog
Possible solution
Replace Mailhog with Mailpit GitHub - axllent/mailpit: An email and SMTP testing tool with API for developers