At least since stunnel 4.15 - but not yet in 4.09 (don't know about the intermediate versions) - there is a (cosmetic only?) bug in the "welcome" forwarding of the smtp_server in protocol.c (lines 177-187 in 4.16): fdgetline(c, c->remote_fd.fd, line); if(!isprefix(line, "220")) { s_log(LOG_ERR, "Unknown server welcome"); longjmp(c->err, 1); } fdprintf(c, c->local_wfd.fd, "220%s + stunnel", line); fdgetline(c, c->local_rfd.fd, line); if(!isprefix(line, "EHLO ")) { s_log(LOG_ERR, "Unknown client EHLO"); longjmp(c->err, 1); } Although the line read from remote_fd.fd already starts with "220", another 220 is prepended in forwarding to local_wfd.fd, line 182. This seems to haveno adverse effects, but this line should better be changed to: fdprintf(c, c->local_wfd.fd, "%s + stunnel", line);