sx SMTP connection, contains buffer for pipelining, and socket
mode buffer, write-with-more-likely, write
format a format, starting with one of
- of HELO, MAIL FROM, RCPT TO, DATA, ".", or QUIT.
+ of HELO, MAIL FROM, RCPT TO, DATA, BDAT, ".", or QUIT.
If NULL, flush pipeline buffer only.
... data for the format