Ok, this is a bit off but wondering if someone has seen this before.

I have a 7400 4.98 svmi20i
The email gateway, about once a day, though not at a consistent time produces this error"

WARNING - SMTP '.' command failed. Server (]server address redacted) Response '550 5.7.1 Invalid sender address. ([email protected])

I have a mailbox that the day before sends a voicemail fine, and the day after, also sends a voicemail without error. Same mailbox. No change in settings.
Consistent SMTP info in sys params, mclass and the email gateway for the mailbox itself.

I cannot for the life of me find "[email protected]" in any field anywhere.

It is not always the same mailbox that gets the error.

The client has a smtp relay going so it is on port 25 with no username or password required or used.

Help me hive mind, you're my only hope.