smtp problems on "confirmed working" smtp server

[Demian Smith <demian.smith@web.de>]

Hi List,

I hope I did read the documentation sufficiently and have come to the
right place with this request - if not, please don't hesitate telling me
off. Unfortunately, forums.debian.net seems dead for 2 years, otherwise
I would have posted there.

Since a couple of days I cannot send email with one of my smtps
(mail.riseup.net) and have additional problems with other smtps. For the
other smtps I can send messages, unless I attach any file to the mail.
The main issue is the smtp of riseup failing completely with the message:
"Sending of the message failed.
The message could not be sent using Outgoing server (SMTP)
mail.riseup.net for an unknown reason. Please verify that your Outgoing
server (SMTP) settings are correct and try again."

I have confirmed with riseup that smtp is up and running and have, in
fact, added the same account with the same details in a different OS
where I don't see problems sending mails.

I have tested both ssl/tls and STARTTLS on all port (25, 465 and 578),
all without any success. I suspected a DNS error first, but verified
that my DNS resolves ok for the host, so I am now expecting that maybe
some update did mess something on my system, but am not able to
determine which update / which setting.

The last upgrades I did were as follows: https://paste.debian.net/683466

I did try a reinstall of icedove to no avail and also confirmed the same
issue with kmail on the same system.

I am running Debian 8.4:

$ lsb_release -da
LSB Version:
core-2.0-amd64:core-2.0-noarch:core-3.0-amd64:core-3.0-noarch:core-3.1-amd64:core-3.1-noarch:core-3.2-amd64:core-3.2-noarch:core-4.0-amd64:core-4.0-noarch:core-4.1-amd64:core-4.1-noarch:security-4.0-amd64:security-4.0-noarch:security-4.1-amd64:security-4.1-noarch
Distributor ID: Debian
Description:    Debian GNU/Linux 8.4 (jessie)
Release:        8.4
Codename:       jessie

$ konsole --version
KDE is KDE Development Platform: 4.14.2

My main mail client is icedove:
$ icedove --version
Icedove 38.7.0

but I tested the settings as well in KMail 4.14.1


And I have www.deb-multimedia.org enabled in /etc/apt/sources.list.d/


So, as you can see from my stumbling in the dark, I am totally lost for
this one and any help or pointers would be appreciated, before I go and
nuke the system for a reinstall...

Thanks in advance,
Demian