Redmine

Hi

We have very strange problem with email notification.
Redmine stops sending email after random time.
We have to restart nginx to solve problem temporary till next time users report that they do not receiving mails.
We did upgrade from 2.4 to 2.6.0. We also upgraded os to ubuntu 14.0.4 but it did not solve a problem.
Redmine uses sendmail in configuration.
We also examined logs but found nothing.

nginx version: nginx/1.4.6 (Ubuntu)
Using unicorn (4.8.3)
mail_version = 2.11.0
Environment:
Redmine version 2.6.0.stable
Ruby version 1.9.3-p484 (2013-11-22) [x86_64-linux]
Rails version 3.2.19
Environment production
Database adapter Mysql2
SCM:
Git 1.9.1
Filesystem
Redmine plugins:
redmine_helpdesk 0.0.8
redmine_issue_due_date 0.2.0
redmine_tags 2.0.1-dev