Q: Resend email button doesn't resend the email, why?
A: Most probably because Resend retention time is not set to 30 or 90 days.
Please go to your Domain Settings > Additional domain settings and check if you have set the RESEND RETENTION TIME. If it is set to none, that is why you cannot resend an email.
For more details regarding Heimdal™ E-Mail Security, click here: https://support.heimdalsecurity.com/hc/en-us/articles/360007381137E-Mail-Security-and-Spamfilter-Configuration-