TLS socket closed (SSL_write) with unsent byte(s)

Hi All,

We use stunnel to send emails from our scanners to our hosted mail provider (that now only accept TLS connections).
This has been working well for a number of years, but just recently the scanners (all Lexmark) have sporadically been giving errors of 'connection failed'.

If I check the stunnel logs, the failed send has an extra line in of TLS socket closed (SSL_write) with 418 unsent byte(s)

Below are 2 examples, 59 was sent successfully, 60 was not.

2018.01.16 14:37:24 LOG5[59]: Service [fasthosts-smtp] accepted connection from 10.226.174.95:11473
2018.01.16 14:37:24 LOG5[59]: s_connect: connected 213.171.216.50:465
2018.01.16 14:37:24 LOG5[59]: Service [fasthosts-smtp] connected remote server from 10.226.150.9:52577
2018.01.16 14:37:24 LOG5[59]: OCSP: Connecting the AIA responder "http://t2.symcb.com"
2018.01.16 14:37:24 LOG5[59]: s_connect: connected 23.55.155.27:80
2018.01.16 14:37:24 LOG5[59]: OCSP: Certificate accepted
2018.01.16 14:37:24 LOG5[59]: OCSP: Connecting the AIA responder "http://tj.symcd.com"
2018.01.16 14:37:25 LOG5[59]: s_connect: connected 23.55.155.27:80
2018.01.16 14:37:25 LOG5[59]: OCSP: Certificate accepted
2018.01.16 14:37:25 LOG5[59]: Certificate accepted at depth=0: C=GB, ST=Gloucestershire, L=Gloucester, O=Fasthosts Internet Limited, OU=NOC, CN=smtp.livemail.co.uk
2018.01.16 14:37:29 LOG5[59]: Connection closed: 167106 byte(s) sent to TLS, 376 byte(s) sent to socket

2018.01.16 14:38:19 LOG5[60]: Service [fasthosts-smtp] accepted connection from 10.226.174.95:11483
2018.01.16 14:38:19 LOG5[60]: s_connect: connected 213.171.216.50:465
2018.01.16 14:38:19 LOG5[60]: Service [fasthosts-smtp] connected remote server from 10.226.150.9:52620
2018.01.16 14:38:19 LOG5[60]: OCSP: Connecting the AIA responder "http://t2.symcb.com"
2018.01.16 14:38:19 LOG5[60]: s_connect: connected 23.55.155.27:80
2018.01.16 14:38:19 LOG5[60]: OCSP: Certificate accepted
2018.01.16 14:38:19 LOG5[60]: OCSP: Connecting the AIA responder "http://tj.symcd.com"
2018.01.16 14:38:19 LOG5[60]: s_connect: connected 23.55.155.27:80
2018.01.16 14:38:19 LOG5[60]: OCSP: Certificate accepted
2018.01.16 14:38:19 LOG5[60]: Certificate accepted at depth=0: C=GB, ST=Gloucestershire, L=Gloucester, O=Fasthosts Internet Limited, OU=NOC, CN=smtp.livemail.co.uk
2018.01.16 14:38:24 LOG3[60]: TLS socket closed (SSL_write) with 418 unsent byte(s)
2018.01.16 14:38:24 LOG5[60]: Connection reset: 964 byte(s) sent to TLS, 392 byte(s) sent to socket

Has anyone come across this before?
Our mail provider has send limits in place of 50 every 10 minutes, but I am confident we are not reaching those amounts.

Any help appreciated.

Regards, Stephen

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