Thunderbird 60.5.2 causes a MAPI error – a workaround

[German]Mozilla's developers released the e-mail client Thunderbird version 60.5.2 on February 25, 2019. This maintenance update should fix some bugs, but caused users to get a MAPI error when trying to send mail attachments. It is no longer possible to send files via MAPI (possibly from third party applications via mailto).


I had briefly reported about the new version in the blog post Thunderbird 60.5.2 released. I mentioned a correction to the certificate check for e-mails sent through Outlook. So it seems, that the MAPI interface has been altered.

MAPI error: Can't send files anymore

In the comments to the German blog post Thunderbird 60.5.2 verfügbar some users reported, that Thunderbird 60.5.2 can't send any files no more. A user wrote:

I noticed a problem with Mozilla Thunderbird 60.5.2 on 4 computers. If the update is installed from within the program [auto update], no more files can be sent by e-mail from within an application or from the desktop.

The user states that he did not have this problem with the previous version. In the comments other users confirm this bug – and one reports that the message:

Error: An error occurred when calling a MAPI function…..

is displayed. Such an error already existed in 2009 in Thunderbird (see here), but has another root cause. In the German Thunderbird Mail-Forum there is this thead that's addressed the bug.

The bug is known to the developers

In the Thunderbird support forum there is since February 27, 2019 this entry, which describes an issue with scanning of documents and send it as e-mail attachment. A user wrote, that his HP scan application is no longer able to send the scanned documents via e-mail client. Within the thread as well as in this German comment by blog reader Joachim (thanks for that) it is confirmed that the MAPI function in Thunderbird 60.5.2 is broken.


Bugzilla has a bug report 1531869 (Automatic update does not replace MAPI DLLs), which describes the cause of this issue very precisely. During the automatic update installation, the installer cannot completely replace all MAPI DLLs under Windows.

Before the update: 60.5.1 DLLs:
C:\Program Files\Mozilla Thunderbird 60\MapiProxy_InUse.dll – 22 KB – 2019/02/13
C:\Program Files\Mozilla Thunderbird 60\mozMapi32_InUse.dll – 97 KB – 2019/02/13

Identical files MapiProxy.dll and mozMapi32.dll

After update to 60.5.2, these are still there:
C:\Program Files\Mozilla Thunderbird 60\MapiProxy_InUse.dll – 22 KB – 2019/02/13
C:\Program Files\Mozilla Thunderbird 60\mozMapi32_InUse.dll – 97 KB – 2019/02/13

And also:
C:\Program Files\Mozilla Thunderbird 60\mozMapi32.dll – 96 KB – 2019/03/01
C:\Program Files\Mozilla Thunderbird 60\MapiProxy.dll – 22 KB – 2019/03/01

The date 2019/03/01 is the date of the auto-update. The Thunderbird program folder contains the new versions of the DLLs, but the "InUse" version could not be updated. This results in a MAPI error, because the old DLLs doesn't fits the new program version anymore.

This bug is mentioned now within the release notes. Within the release notes there is a second bug mentioned: Files with non-ASCII characters in their name also cause a malfunction. There is also a workaround for this issue proposed within the release notes.

A simple workaround usually helps

The solution to this issues has been mentioned within the comments of my German blog post about Thunderbird 60.5.2: Simply uninstall the Thunderbird email client completely and then reinstall it in the next step. This has helped for most people. But there are also comments within my German blog post, saying that this workaround did not help on all systems. I guess that one of these DLLs was still in use and was therefore not uninstalled. Here you should restart Windows to avoid such problems. If that doesn't work, you could manually check the above paths and see if the DLLs are still there.

Blog reader Joachim writes in this comment that he installed the old Thunderbird 60.5.0 over the new version (disable the auto-update function via the help menu and settings before installing the older version). After that the mail dispatch from other programs worked again. In this case, users using the Lightning calendar should manually reset it to version 6.2.4. However, the 'reinstall' approach to the Thunderbird client did not help all affected users. Possibly the MAPI settings in the registry were corrupted when the update failed.

Addendum: Thunderbird 60.5.2 has been pulled from auto-update, seeThunderbird 60.5.2 pulled (March 5, 2019).

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