The reason was that all links in the newsletter were redirected via my server in order to be counted by the statistic module of my newsletter software. If a lot of readers try to access at the same time it becomes critical.
I have found a way to deactivate the statistic feature. Links in the newsletter now go directly to the final web page which most of the time is not on my server.
In addition I will try to avoid to distribute the newsletter during business hours from now on. Therefore not all readers will access at the same time.