SOLVED!
It seems as there is a bug in EmailSender.h file when using STORAGE_SDFAT2 option: using SdFat library with ESP8266, you have to edit line 323 of this file in the section ‘#elif (EXTERNAL_STORAGE == STORAGE_SDFAT2)’
from ‘extern SdFat EXTERNAL_STORAGE_CLASS;’
to ‘extern SdFs EXTERNAL_STORAGE_CLASS;’
For others interested to use this wonderful library to build small devices that must log data in the SD and send a file by email, my settings in file EMailSenderKey.h from default must be set as follows:
#define SD_CS_PIN D4 // GPIO pin connected to my SD CS pin
#define DEFAULT_EXTERNAL_ESP8266_STORAGE STORAGE_SDFAT2
#define DEFAULT_EXTERNAL_ESP32_STORAGE STORAGE_SDFAT2
#define DEFAULT_INTERNAL_ESP8266_STORAGE STORAGE_NONE
#define DEFAULT_INTERNAL_ESP32_STORAGE STORAGE_NONE
#define STORAGE_INTERNAL_FORCE_DISABLE // if internal storage is not needed
With this, it works as charm and uses much less resources to send emails than other similar libraries.
Thank you, Renzo!