354
SMTP
354 Start Mail Input
The server is ready to receive the message body. The client should begin sending the email content (headers and body) and terminate with a single line containing only a period (CRLF.CRLF).
이 코드를 보게 되는 경우
After sending the DATA command. The server is now waiting for you to type or stream the email message, ending with a dot on its own line.
해결 방법
Send your message content followed by a line with just a period (.) to signal the end. If you get an error after this, check that your message does not contain a bare period on a line by itself within the body.