External email submission results in a null sender

so i’m testing my RT’s email. we use an onprem imap server. i have RT configured to getmail and to msmtp.
it works fine when sending/receiving to/from internal email addresses, but not to receive emails sent externally.

getmail seems to retrieve it, but it doesn’t generate a ticket, although it tries to send a reply… without a “from” address, so iredmail blocks it.
not sure why this is an issue, and why it wouldn’t have a from address.