If i send an invalid emailaddress via SMTP to gmail, after mailFrom and rcptTo. If rcptTo address is invalid, gmail sends 550 User not found. But yahoo is sending user not found only after DATA part is send. Is there any way to find email validity before DATA part
Validating user after rcpto send is not mandatory. It is up to developers choice whether to validate user before DATA part or after. Please refer to this https://www.freesoft.org/CIE/RFC/1123/92.htm If you want to validate user before DATA, you can use third party email validators and clean your contact list and then send for only validate users