Sorry, I have no idea what Debian version or Kolab it was. Liutas lost 2 hours investigating a Roundcube issue in Debian. We found out this was a bug in Net_Socket library. This one: https://github.com/pear/Net_Socket/commit/c8d2ea02d50dc65e33bc0ec37c6dd121a3e06148
A workaround for this issue was to set smtp_timeout to value greater than 0 in Roundcube config.
The php-net-socket package version was 1.0.9-2, which is 7 years old. Please, provide and require at least 1.0.12 (better 1.0.14), which includes the bugfix.
Jeroen, please assign appropriate tags. Finding distribution tags is not simple when you don't know code names.