Unable to send email via JavaMail

Prime Messaging Exception
501 Error: Syntax: HELO hostname

As far as I can tell we should only get that error when the SMTP hostname hasn't been set correctly, but it has been set (we can see it in the UI, and I can find it in the database as well). We've also verified that we can send email through resend using this exact SMTP configuration using swaks (recommended here).

swaks --from '<FROM_EMAIL>' --to '<TO_EMAIL>' --server smtp.resend.com --port 587 --auth plain --tls --auth-user 'resend' --auth-password '<RESEND_API_KEY>'

^ this works perfectly.

I have FusionAuth SMTP debugging enabled, and this is all we see when sending a test email:

DEBUG: Jakarta Mail version 2.1.2
DEBUG: URL jar:file:/usr/local/fusionauth/fusionauth-app/lib/smtp-2.0.2.jar!/META-INF/javamail.providers
DEBUG: successfully loaded resource: jar:file:/usr/local/fusionauth/fusionauth-app/lib/smtp-2.0.2.jar!/META-INF/javamail.providers
DEBUG: Tables of loaded providers
DEBUG: Providers Listed By Class Name: {org.eclipse.angus.mail.smtp.SMTPTransport=jakarta.mail.Provider[TRANSPORT,smtp,org.eclipse.angus.mail.smtp.SMTPTransport,Oracle], org.eclipse.angus.mail.smtp.SMTPSSLTransport=jakarta.mail.Provider[TRANSPORT,smtps,org.eclipse.angus.mail.smtp.SMTPSSLTransport,Oracle]}
DEBUG: Providers Listed By Protocol: {smtp=jakarta.mail.Provider[TRANSPORT,smtp,org.eclipse.angus.mail.smtp.SMTPTransport,Oracle], smtps=jakarta.mail.Provider[TRANSPORT,smtps,org.eclipse.angus.mail.smtp.SMTPSSLTransport,Oracle]}
DEBUG: successfully loaded resource: /META-INF/javamail.default.address.map
DEBUG: URL jar:file:/usr/local/fusionauth/fusionauth-app/lib/smtp-2.0.2.jar!/META-INF/javamail.address.map
DEBUG: successfully loaded resource: jar:file:/usr/local/fusionauth/fusionauth-app/lib/smtp-2.0.2.jar!/META-INF/javamail.address.map
DEBUG: setDebug: Jakarta Mail version 2.1.2