This library use the SMTP:
The Simple Mail Transfer Protocol (SMTP) is a communication protocol for electronic mail transmission. As an Internet standard, SMTP was first defined in 1982 by RFC 821, and updated in 2008 by RFC 5321 to Extended SMTP additions, which is the protocol variety in widespread use today. Mail servers and other message transfer agents use SMTP to send and receive mail messages. Proprietary systems such as Microsoft Exchange and IBM Notes and webmail systems such as Outlook.com, Gmail and Yahoo! Mail may use non-standard protocols internally, but all use SMTP when sending to or receiving email from outside their own systems. SMTP servers commonly use the Transmission Control Protocol on port number 25. (cit WiKi)
Library
You can find my library here, or just search for EMailSender on the Arduino IDE library manager.
Arduino (support W5100 like must be set, and ENC28J60 via UIPEthernet), esp8266 (SPIFFS and SD) (core <=2.4.2 must be set) and esp32 (SPIFFS and SD), Arduino WiFiNINA SAMD devices (Arduino MKR WiFi 1010, Vidor 4000 UNO WiFi Rev.2 ecc.) (SD).
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