Sendmail email notifications don't contain a message body

Hello, I have gitea setup through the official docker image and I’ve been succesful in sending emails using sendmail, BUT the emails only contain a subject line, the message body is blank.

When I login to the docker container through the command line to test out sendmail manually, it works BUT I am only able to see a message body if I have a between the subject and the message body, like this:

$ sendmail -S host
Subject: This is the subject

I need to leave a space between the subject and the message body for this email to work

Is this a common issue with sendmail? For reference, here is my app.ini:

ENABLED = true
MAILER_TYPE = sendmail
SENDMAIL_PATH = /usr/sbin/sendmail