Here are some samples - payload1.json and payload2.json fail with the errors below, but payload3.json (also containing non-ASCII characters in content.from.name receives no errors):
curl http://localhost:8000/api/inject/v1 -X POST -H 'Content-Type: application/json' -d @payload1.json
{"success_count":0,"fail_count":1,"failed_recipients":["hf.farhad@gmail.com"],"errors":["hf.farhad@gmail.com: invalid header: 0: at line 1:\n=?UTF-8?q?=D8=B1=D9=87=D9=86=D9=85=D8=A7_=DA=A9=D8=A7=D9=84=D8=AC?= <no-␍\n ^______________________________\nexpected ':', found <\n\n1: at line 1, in group:\n=?UTF-8?q?=D8=B1=D9=87=D9=86=D9=85=D8=A7_=DA=A9=D8=A7=D9=84=D8=AC?= <no-␍\n^__________________________________________________________________________________________________\n\n2: at line 1, in Alt:\n=?UTF-8?q?=D8=B1=D9=87=D9=86=D9=85=D8=A7_=DA=A9=D8=A7=D9=84=D8=AC?= <no-␍\n^__________________________________________________________________________________________________\n\n3: at line 1, in address:\n=?UTF-8?q?=D8=B1=D9=87=D9=86=D9=85=D8=A7_=DA=A9=D8=A7=D9=84=D8=AC?= <no-␍\n^__________________________________________________________________________________________________\n\n4: at line 1, in Many1:\n=?UTF-8?q?=D8=B1=D9=87=D9=86=D9=85=D8=A7_=DA=A9=D8=A7=D9=84=D8=AC?= <no-␍\n^__________________________________________________________________________________________________\n\n5: at line 1, in obs_address_list:\n=?UTF-8?q?=D8=B1=D9=87=D9=86=D9=85=D8=A7_=DA=A9=D8=A7=D9=84=D8=AC?= <no-␍\n^__________________________________________________________________________________________________\n\n6: at line 1, in Alt:\n=?UTF-8?q?=D8=B1=D9=87=D9=86=D9=85=D8=A7_=DA=A9=D8=A7=D9=84=D8=AC?= <no-␍\n^__________________________________________________________________________________________________\n\n7: at line 1, in address_list:\n=?UTF-8?q?=D8=B1=D9=87=D9=86=D9=85=D8=A7_=DA=A9=D8=A7=D9=84=D8=AC?= <no-␍\n^__________________________________________________________________________________________________\n\n\nstack traceback:\n\t[C]: in method 'from_header'\n\t/opt/kumomta/etc/policy/dkim_sign.lua:244: in upvalue 'do_dkim_sign'\n\t/opt/kumomta/etc/policy/dkim_sign.lua:423: in upvalue 'dkim_signer'\n\t[string \"/opt/kumomta/etc/policy/init.lua\"]:454: in function <[string \"/opt/kumomta/etc/policy/init.lua\"]:370>\n"]}
curl http://localhost:8000/api/inject/v1 -X POST -H 'Content-Type: application/json' -d @payload2.json
{"success_count":0,"fail_count":1,"failed_recipients":["hf.farhad@gmail.com"],"errors":["hf.farhad@gmail.com: invalid header: 0: at line 1:\n=?UTF-8?q?=D8=A2=D9=87=D8=A7_=D8=B3=D9=86=D8=AF?= <no-␍\n ^______________________________\nexpected ':', found <\n\n1: at line 1, in group:\n=?UTF-8?q?=D8=A2=D9=87=D8=A7_=D8=B3=D9=86=D8=AF?= <no-␍\n^________________________________________________________________________________\n\n2: at line 1, in Alt:\n=?UTF-8?q?=D8=A2=D9=87=D8=A7_=D8=B3=D9=86=D8=AF?= <no-␍\n^________________________________________________________________________________\n\n3: at line 1, in address:\n=?UTF-8?q?=D8=A2=D9=87=D8=A7_=D8=B3=D9=86=D8=AF?= <no-␍\n^________________________________________________________________________________\n\n4: at line 1, in Many1:\n=?UTF-8?q?=D8=A2=D9=87=D8=A7_=D8=B3=D9=86=D8=AF?= <no-␍\n^________________________________________________________________________________\n\n5: at line 1, in obs_address_list:\n=?UTF-8?q?=D8=A2=D9=87=D8=A7_=D8=B3=D9=86=D8=AF?= <no-␍\n^________________________________________________________________________________\n\n6: at line 1, in Alt:\n=?UTF-8?q?=D8=A2=D9=87=D8=A7_=D8=B3=D9=86=D8=AF?= <no-␍\n^________________________________________________________________________________\n\n7: at line 1, in address_list:\n=?UTF-8?q?=D8=A2=D9=87=D8=A7_=D8=B3=D9=86=D8=AF?= <no-␍\n^________________________________________________________________________________\n\n\nstack traceback:\n\t[C]: in method 'from_header'\n\t/opt/kumomta/etc/policy/dkim_sign.lua:244: in upvalue 'do_dkim_sign'\n\t/opt/kumomta/etc/policy/dkim_sign.lua:423: in upvalue 'dkim_signer'\n\t[string \"/opt/kumomta/etc/policy/init.lua\"]:454: in function <[string \"/opt/kumomta/etc/policy/init.lua\"]:370>\n"]}
curl http://localhost:8000/api/inject/v1 -X POST -H 'Content-Type: application/json' -d @pakat-kumo-payload.json
{"success_count":1,"fail_count":0,"failed_recipients":[],"errors":[]}