I want to learn, how can I send an email to gmail domain using smtp protocol and with w5100clon ethernet shield. Please someone can help me on this?
I search many web pages but I couldn't find correct code. My project gives an error. it says connected but later timeout. I change user name and password using base64 but still I have still same problem.
gmail uses secure protocol. you would need to store the public key and calculate the security hashes. AVR can’t do it (Uno WiFi Rev 2 will, with support of external chip), but with samd board it could be possible.
Gmail allow unsafe applications to mail send changing a setting. I can connect to smtp server but while mail sending it fails.
Gmail allows sending a mail to a gmail account using standard SMTP protocol but only if you send it from a fixed IP address. As that is not given for the vast majority of Arduinos out in the wild you will fail in the middle of the SMTP conversation.
So Juraj is correct with he's answer. You need to authenticate to send emails and on port 25 you cannot authenticate on gmail.