After a bit of experimentation, I've succeeded in getting stunnel 5.31 running on my Windows 2008 r2 server and sending email through SMTP through Office365. My problem now is that it's quite slow. I can see a point in the log file where it stops for 5 seconds or more for each email message. Are there any parameter for stunnel that might help, or do I have to just accept that Office365 is slow? Here is my stunnel.conf file:

debug = 7 output = stunnel.log [ms-smtp] protocol = smtp client = yes accept = 587 connect = smtp.office365.com:587

And here is the log file for an email transmission. Notice the 5 second pause at 12:31:04. Is there a way to eliminate this pause?

2016.03.01 12:30:21 LOG7[cron]: Cron thread initialized 2016.03.01 12:30:21 LOG7[main]: No limit detected for the number of clients 2016.03.01 12:30:21 LOG5[main]: stunnel 5.31 on x86-pc-msvc-1500 platform 2016.03.01 12:30:21 LOG5[main]: Compiled/running with OpenSSL 1.0.2g-fips 1 Mar 2016 2016.03.01 12:30:21 LOG5[main]: Threading:WIN32 Sockets:SELECT,IPv6 TLS:ENGINE,FIPS,OCSP,PSK,SNI 2016.03.01 12:30:21 LOG7[main]: errno: (*_errno()) 2016.03.01 12:30:21 LOG7[service]: GUI message loop initialized 2016.03.01 12:30:21 LOG5[main]: Reading configuration from file stunnel.conf 2016.03.01 12:30:21 LOG5[main]: UTF-8 byte order mark detected 2016.03.01 12:30:21 LOG5[main]: FIPS mode disabled 2016.03.01 12:30:21 LOG7[main]: Compression disabled 2016.03.01 12:30:21 LOG7[main]: Snagged 64 random bytes from C:/.rnd 2016.03.01 12:30:21 LOG7[main]: Wrote 1024 new random bytes to C:/.rnd 2016.03.01 12:30:21 LOG7[main]: PRNG seeded successfully 2016.03.01 12:30:21 LOG6[main]: Initializing service [ms-smtp] 2016.03.01 12:30:21 LOG7[main]: No certificate or private key specified 2016.03.01 12:30:21 LOG4[main]: Service [ms-smtp] needs authentication to prevent MITM attacks 2016.03.01 12:30:21 LOG7[main]: SSL options: 0x03000004 (+0x03000000, -0x00000000) 2016.03.01 12:30:21 LOG5[main]: Configuration successful 2016.03.01 12:30:21 LOG7[main]: Listening file descriptor created (FD=424) 2016.03.01 12:30:21 LOG7[main]: Service [ms-smtp] (FD=424) bound to 0.0.0.0:587 2016.03.01 12:30:58 LOG7[main]: Found 1 ready file descriptor(s) 2016.03.01 12:30:58 LOG7[main]: FD=388 ifds=r-x ofds=--- 2016.03.01 12:30:58 LOG7[main]: Service [ms-smtp] accepted (FD=440) from 192.168.8.58:55266 2016.03.01 12:30:58 LOG7[main]: Creating a new thread 2016.03.01 12:30:58 LOG7[main]: New thread created 2016.03.01 12:30:58 LOG7[0]: Service [ms-smtp] started 2016.03.01 12:30:58 LOG5[0]: Service [ms-smtp] accepted connection from 192.168.8.58:55266 2016.03.01 12:30:58 LOG6[0]: failover: round-robin, starting at entry #3 2016.03.01 12:30:58 LOG6[0]: s_connect: connecting 132.245.34.194:587 2016.03.01 12:30:58 LOG7[0]: s_connect: s_poll_wait 132.245.34.194:587: waiting 10 seconds 2016.03.01 12:30:58 LOG5[0]: s_connect: connected 132.245.34.194:587 2016.03.01 12:30:58 LOG5[0]: Service [ms-smtp] connected remote server from 192.168.8.14:55477 2016.03.01 12:30:58 LOG7[0]: Remote descriptor (FD=460) initialized 2016.03.01 12:30:59 LOG7[0]: <- 220 CY1PR21CA0008.outlook.office365.com Microsoft ESMTP MAIL Service ready at Tue, 1 Mar 2016 20:30:58 +0000 2016.03.01 12:30:59 LOG7[0]: -> 220 CY1PR21CA0008.outlook.office365.com Microsoft ESMTP MAIL Service ready at Tue, 1 Mar 2016 20:30:58 +0000 2016.03.01 12:30:59 LOG7[0]: -> EHLO localhost 2016.03.01 12:30:59 LOG7[0]: <- 250-CY1PR21CA0008.outlook.office365.com Hello [#.#.#.#] 2016.03.01 12:30:59 LOG7[0]: <- 250-SIZE 157286400 2016.03.01 12:30:59 LOG7[0]: <- 250-PIPELINING 2016.03.01 12:30:59 LOG7[0]: <- 250-DSN 2016.03.01 12:30:59 LOG7[0]: <- 250-ENHANCEDSTATUSCODES 2016.03.01 12:30:59 LOG7[0]: <- 250-STARTTLS 2016.03.01 12:30:59 LOG7[0]: <- 250-8BITMIME 2016.03.01 12:30:59 LOG7[0]: <- 250-BINARYMIME 2016.03.01 12:30:59 LOG7[0]: <- 250 CHUNKING 2016.03.01 12:30:59 LOG7[0]: -> STARTTLS 2016.03.01 12:30:59 LOG7[0]: <- 220 2.0.0 SMTP server ready 2016.03.01 12:30:59 LOG6[0]: SNI: sending servername: smtp.office365.com 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): before/connect initialization 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): SSLv2/v3 write client hello A 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): SSLv3 read server hello A 2016.03.01 12:30:59 LOG6[0]: Certificate verification disabled 2016.03.01 12:30:59 LOG6[0]: Certificate verification disabled 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): SSLv3 read server certificate A 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): SSLv3 read server key exchange A 2016.03.01 12:30:59 LOG6[0]: Empty client CA list 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): SSLv3 read server certificate request A 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): SSLv3 read server done A 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): SSLv3 write client certificate A 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): SSLv3 write client key exchange A 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): SSLv3 write change cipher spec A 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): SSLv3 write finished A 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): SSLv3 flush data 2016.03.01 12:30:59 LOG7[0]: SSL state (connect): SSLv3 read finished A 2016.03.01 12:30:59 LOG7[0]: 1 client connect(s) requested 2016.03.01 12:30:59 LOG7[0]: 1 client connect(s) succeeded 2016.03.01 12:30:59 LOG7[0]: 0 client renegotiation(s) requested 2016.03.01 12:30:59 LOG7[0]: 0 session reuse(s) 2016.03.01 12:30:59 LOG6[0]: SSL connected: new session negotiated 2016.03.01 12:30:59 LOG7[0]: Peer certificate was cached (4882 bytes) 2016.03.01 12:30:59 LOG6[0]: Negotiated TLSv1.2 ciphersuite ECDHE-RSA-AES256-SHA384 (256-bit encryption) 2016.03.01 12:30:59 LOG7[0]: Compression: null, expansion: null 2016.03.01 12:31:04 LOG6[0]: Read socket closed (readsocket) 2016.03.01 12:31:04 LOG7[0]: Sending close_notify alert 2016.03.01 12:31:04 LOG7[0]: SSL alert (write): warning: close notify 2016.03.01 12:31:04 LOG6[0]: SSL_shutdown successfully sent close_notify alert 2016.03.01 12:31:04 LOG6[0]: SSL socket closed (SSL_read) 2016.03.01 12:31:04 LOG7[0]: Sent socket write shutdown 2016.03.01 12:31:04 LOG5[0]: Connection closed: 413 byte(s) sent to SSL, 635 byte(s) sent to socket 2016.03.01 12:31:04 LOG7[0]: Remote descriptor (FD=460) closed 2016.03.01 12:31:04 LOG7[0]: Local descriptor (FD=440) closed 2016.03.01 12:31:04 LOG7[0]: Service [ms-smtp] finished (0 left)

Thanks very much, Andy