Problem Description

When digital security certificates are used in emails sent via Microsoft Outlook, it may occur that emails are sent incomplete.

Based on current findings, the issue arises after messages have already been saved as drafts. Starting from the second modification after the initial creation of a draft, changes are no longer saved in the message.

Workaround

For sending emails, use the mail server’s web interface at https://mail.desy.de, where this issue does not occur. In Outlook, you can bypass the problem by disabling the signing of emails with digital security certificates:

1. In Outlook, go to File → Options.
2. In the new window, select Trust Center from the left-hand menu, then click Trust Center Settings.
3. Navigate to Email Security.
4. Uncheck the box labeled "Add digital signature to outgoing messages" and confirm the dialog.
    

If needed, you can also control this on a per-message basis by opening the message window, selecting the Options tab, and clicking the Sign button to enable or disable signing.

Solution

A call has been opened for this problem at the manufacturer (Zimbra). The issue has since been confirmed as a bug by the manufacturer (ZBUG-5437). A fix is currently being worked on.

Problem Description

In Outlook, you can select which signature is automatically appended below the message text by default. It has been observed that Outlook "forgets" this setting, resulting in no text signature being automatically added to messages.

Workaround

The signature must be manually added via the message menu.

Solution

Unfortunately, since the observed behavior has not yet (02/2026) been reliably reproducible, we are currently unable to open a support ticket with the vendor for this issue. Should new findings emerge, we will re-examine the matter.

Problem Description

In text signatures, special characters (e.g., umlauts like ä, ö, ü) are replaced with "broken" or corrupted characters, resulting in incorrect display.

Workaround

Disable the synchronization of signatures via the Zimbra Connector for Outlook (ZCO) as follows:

1. Open Outlook.
2. In the menu, go to Zimbra → Advanced.
3. Uncheck the box labeled "Synchronize signatures" and confirm the dialog.
    

Any previously corrupted signatures must be recreated. Disabling this option prevents them from being synchronized to the server, which should prevent them from becoming corrupted again.

Note: Disabling signature synchronization means that signatures created in Outlook will not automatically appear in the Zimbra web interface (https://mail.desy.de). If needed, they must be created separately there.

Solution

Since the update to the latest Zimbra version (10.1) on 12.02.2026, the issue could no longer be observed during testing. It is therefore possible that the update resolved this problem. We will continue to monitor this for some time and will provide updates here if any new findings emerge.

Problem Description

HTML signatures created in Zimbra that are synchronized with Outlook via ZCO (Zimbra Connector for Outlook) and potentially edited in Outlook show very large paragraph spacing after a short time. This makes it appear as if empty lines have been inserted between the text lines in the signature.

Workaround

Disable the synchronization of signatures via the Zimbra Connector for Outlook (ZCO) as follows:

1. Open Outlook.
2. In the menu, go to Zimbra → Advanced.
3. Uncheck the box labeled "Synchronize signatures" and confirm the dialog.
    

Any previously corrupted signatures must be recreated. Disabling this option prevents them from being synchronized to the server, which should prevent them from becoming corrupted again.

Note: Disabling signature synchronization means that signatures created in Outlook will not automatically appear in the Zimbra web interface (https://mail.desy.de). If needed, they must be created separately there.

Solution

A request for problem resolution has been submitted to the vendor (Zimbra).

Problem Description

When an email containing embedded images sent from Apple Mail is received in Outlook with ZCO (Zimbra Connector for Outlook), the images are displayed as attachments instead of being embedded in the text. Additionally, all text below the image is also moved to the attachments and removed from the message body.

Workaround

View the email in the Zimbra web interface (https://mail.desy.de), where the display is correct. If needed, use the web interface to reply to emails of this type.

Lösung

A request for problem resolution has been submitted to the vendor (Zimbra).

Problem Description

Occasionally, individual appointments — or sometimes entire recurring series — are displayed twice or even multiple times in Outlook.

In some cases, this is purely a display issue in Outlook. You can verify this by logging in to https://mail.desy.de and checking the appointment there. If it appears only once in the Zimbra web interface, the duplication is merely a display issue in Outlook.
In rare cases, the appointments are actually duplicated and appear multiple times in the affected calendar, even in the Zimbra web interface.

Workaround

Since the duplicates generally do not cause actual problems, the phenomenon can usually be ignored.

If it becomes particularly disruptive in individual cases, the cleanest solution is to delete the appointment and recreate it. Over time, duplicates may accumulate, so it is generally recommended to limit recurring appointments to a maximum of six months and, if necessary, recreate the series for another six months after it expires.

Solution

Unfortunately, since the observed behavior has not yet (02/2026) been reliably reproducible, we are currently unable to open a support ticket with the vendor for this issue. If new insights become available, we will re-examine the matter.

Problem Description

In very rare cases, parts of the email header have been observed to appear in the email body after sending, distorting the message content. This issue occurs when:

1. A signed email is initially composed in plain-text format in Outlook,
2. The email is then modified and saved in Zimbra without a digital signature, and
3. The email is subsequently edited and re-signed in Outlook
    

Workaround

Use only Outlook or the Zimbra web interface (https://mail.desy.de) for sending emails that are highly important and must not be affected by this issue.

Solution

A request for problem resolution has been submitted to the vendor (Zimbra).

Problem Description

Appointments assigned to a category with special characters in its name appear in Outlook but not in the Zimbra web interface (https://mail.desy.de).

Background: The current version of Zimbra does not support special characters in category names, whereas Outlook allows special characters in category names. Appointments with such categories are not synchronized.

Workaround

Remove the special characters from the names of the affected categories in Outlook.

Solution

A request for problem resolution has been submitted to the vendor (Zimbra). This has been acknowledged as a Request for Enhancement (02/2026). The behavior will be addressed in a future version of the product.

Problem Description

When composing emails, recipients occasionally disappear from the recipient list. This issue is visible not only in Outlook but potentially in other email clients as well, and it occurs sporadically. The problem becomes apparent, for example, when saving drafts or after sending an email, when reviewing the sent message.

Workaround

Since this issue has so far only been observed in connection with the use of Google Mail (Gmail) on smartphones, the recommendation is to avoid using Gmail. Instead, consider using alternative email apps such as Samsung Mail or others.

Lösung

Since the issue could not be reliably reproduced and our assessment indicates that the software components from the vendor (Zimbra) are not the root cause of this behavior, we are currently unable to open a support ticket with the manufacturer. Should new findings emerge, we will re-evaluate the matter.