courier pop3 version = 0.76.3-5+deb9u1
rut955-firmware = RUT9XX_R_00.06.08.5 (tested with the 7* firmware too, but this bloody beta and the mobile interface doesn't work (german telekom d1) and so no internet connection and no way to test email to sms)
Since our Debian Update to bullseye the email to sms stop working.
Login is ok, but no emails were read and delete
rut955 show this error (ssh login ....)
POP3 RESPONSE : +OK POP3 clients that break here, they violate STD53.
To google this response show answers contain "very old emailclients"
solution (not good, but ...): We install solid-pop3 simple pop3 server on debian buster (on rasperry pi - only for teltonika ...)