Email inbound process is done by user SAPCONNECT so that you cannot debug it via normal debugging approach.
then you have to follow the steps below to debug:
(1) Inside one entry like below into table BCSD_BREAKLOOP.
BL Module = INBOUND
CREATEDFOR = SAPCONNECT
Set the expire time accordingly.
Note: As the insertion of this entry will lead to an endless loop in SAPconnect inbound processing, please make sure to delete it or set the status to abap_false.
(2) Send your mail to agent inbox mail box as usual, then go to tcode SM51, you should find the endless loop there.
This is because your entry inserted by step1 takes effect:
(3) In debugger, use Shift+F12 to jump out of the endless loop manually:
After that you could debug the left inbound process of SAP connect now: