I had same kind of errors due to CCSID.
MMAIL is tested in a 400 with QCCSID=037 & jobccsid 037
As I have a 400 with qccsid = 65535 & jobccsid = 297 (ie from France), I had the @ char transformed to a (classic conversion error).
So I have found one configuration where message could be sent quite correctly. This configuration is not one of our production configuration, so we don’t use mmail in production.
We will rework mmail when we will go to QCCSID not 65535
have you some dumps ?
have you some strange message before the escape error ?