I have added this class to the WiFiESP library if anyone is interested in using it.
I have tested it and received numerous emails in MS Outlook sent from an Arduino Mega with a Wifi shield.
SMTPClient.cpp (6.87 KB)
SMTPClient.h (1.71 KB)
I believe boylesg has provided a newer version of these files in this thread:
https://forum.arduino.cc/index.php?topic=466716.0
Is that correct boylesg?