Persons with DESY account have different possibilities to access their e-mails and calendars. The applications offered by DESY-IT for accessing your calendars are briefly described below. At the end of this web page you will find links to specific notes of the programs.

Brief overview

Please note that only the Zimbra web interface provides a direct view of your email/calendar account data. All other applications only provide a local image of the data, which means that it has to be copied from/to the calendar/mail service.

Therefore: Please prefer to use the Zimbra web interface ( Then use a locally installed e-mail/calendar program (e.g. Outlook) if

  • the Zimbra web interface cannot be used efficiently (e.g. on smartphones or tablets)
  • you need your data offline (e.g. on business trips)
  • you need extended functions or a link to other programs (such as Word or
    enterprise resource planning)

Comparison of available calendar programs

Calendar program availability

  Zimbra Web Outlook Thunderbird (Lightning) Apple iCal
Windows yes yes yes no
Linux yes no yes no
macOS yes no yes yes
iOS yes no no yes
Android yes no no no


Relevant aspects of the calendar programs

  Zimbra Web Outlook Thunderbird (Lightning) Andere (e.g. iCal)

Supported by DESY-IT

yes yes no limited

Cross-operating system

yes no yes no

Local image (synchronization)

no yes yes yes

Offline availability of data

no yes yes yes

Integration in office-programs

no yes no no

Configuration required on each device

no yes yes yes

Synchronization-problems possible

no yes yes yes


Zimbra Web

The Zimbra web interface ( is the primary management tool recommended by DESY-IT to access calendars, emails and contacts. It can be used from any device that has an Internet program (browser). It does not matter which operating system is used on the device.

Through the Zimbra web interface, you have unlimited access to all the features and settings of your account. Moreover, you do not need to keep any data locally in order to use the web interface, which avoids a possibly lengthy synchronization of data.

Microsoft Outlook

Microsoft Outlook is limited to use on DESY Windows Devices. In addition to Microsoft Outlook itself, the so-called Zimbra Connector for Outlook (ZCO for short) is required for use. Like Microsoft Office, this must also be installed via the Software Shop. The use of Microsoft Outlook is necessary if you need to make your calendars (and e-mails) accessible offline. For example, to be able to access your data while on the road.

However, some of the required settings or approvals cannot be made in Outlook itself, but must be made beforehand via the Zimbra web interface (see above). Outlook also copies your data from the central service to your local device (creating a local image of your data available on the central service). The required copying process (synchronization) requires a corresponding amount of time especially in case of large inboxes and/or many appointments in calendars, on the one hand during the very first synchronization to your device, and on the other hand after longer periods of absence like e.g. vacations.

The fact that the data is stored locally on the device's hard drive also weakens the protection of the data somewhat compared to using the web interface, as this can create an additional point of attack.

Outlook can offer a clearer view of the calendars, especially for people who are confronted with managing many calendars in everyday life. Outlook can also provide integration into Word or Excel, e.g. for serial letters.


Apple devices come with their own calendar applications. Like Outlook or Thunderbird, these applications also only provide an image of your data locally on the device, which means that it has to be copied down from or to the calendar/mail service. With iCal on macOS, there are known issues, especially with more complex configurations such as split calendars or similar constellations such as deputies. The same is true for the Calendar app that comes with iOS.

The use of these programs may not be unproblematic and can lead to synchronization problems. Therefore we generally recommend the use of the Zimbra web interface (see above). The use of iCal is only recommended for very simple use cases (e.g. exclusive access to the own calendar). iCal can therefore not or only very limited supported. Therefore we can not, or only very limited support iCal.

Smartphones / Tablets

Smartphones and tablets also usually come with their own calendar programs and allow many different calendar programs to be installed via the app stores. These also only provide an image of your data locally on the device. DESY-IT does not yet have a basic recommendation for certain applications.

In principle, however, the use of ActiveSync (which includes the synchronization of mails, calendars and contacts) should be primarily used as a synchronization type. Only if synchronization via Active Sync is not possible or desired, mails (with an IMAP account), calendars (with a CalDAV account) and contacts (with a CardDAV account) should be transferred from/to the central service to your device. Due to the wide variety of programs, different devices, etc., DESY-IT can only provide limited support in these cases.

Thunderbird Lightning

Thunderbird Lightning is the calendar module of Thunderbird. Formerly installable as an additional plugin, it has been a fixed part of Thunderbird for some time. Due to various problems that can occur during data transfer from/to Thunderbird Lightning and were also reproducible in tests, we do not recommend using Thunderbird for calendar management and do not support it centrally. Since data loss can occur in the worst case, we strongly advise against using Thunderbird for calendar management.