This Python script parses calendar invite information included in winmail.dat files sent by some Exchange email servers and generates a calendar invite in ICS format.
Have you ever received an email with a mysterious attachment named winmail.dat? If that's the case, it's highly likely that the sender of that message is using Microsoft Outlook and/or a Microsoft Exchange email server and that you are using a different email client (such as Apple Mail) on a computer that isn't running the Windows operating system (such as macOS).
Such winmail.dat files use a proprietary Microsoft format called Transport Neutral Encapsulation Format (TNEF) and can include rich-text formatted email, attached files or calendar invites. Several tools are available to extract data from winmail.dat files, such as Josh Jacob's TNEF's Enough on macOS. However, while TNEF's Enough is able to extract the body of the offending email and any attached files, it is currently not able to extract calendar invitations that are encoded directly within the TNEF payload (as opposed to an attached ICS calendar invite). The Gmail web client usually does a good job extracting this information and can be sometimes used to open calendar invites hidden in winmail.dat files, but can sometimes miss critical information such as accurate meeting date and time.
Fortunately this looks like a relatively rare problem. I work with a variety of clients, several are using Microsoft Exchange and I have no problem opening their emails and calendar invitations on Apple Mail on my macOS machine. I do however have one client for which I consistently am unable to open their calendar invites. They show up as winmail.dat in their emails, and even after opening them in the Gmail client or through TNEF's Enough, I'm not able to add their invites to my calendar and respond to them, which is annoying to say the least.
To resolve this situation, I wrote a short Python script that takes advantage of the tnefparse and icalendar libraries to extract the relevant information from TNEF-encoded winmail.dat attachments and convert it into an ICS invitation file, which I can then open normally in my calendar app.
This is a quick and dirty solution that works for me for that particular client, but hasn't been tested on other winmail.dat files. Also ideally this functionality could be included in the tnefparse library and its command-line utility, but I haven't yet figured out how to do it. Once I do, I intend to submit a pull request to tnefparse.
As stated above, this script relies on two Python libraries:
These consequently need to be installed on your machine if you want to be able to run this script. This can be done for example by using pip:
pip install tnefparse
pip install icalendar
The script expects at least one argument, which is a path to the input winmail.dat file:
python tnef2ics.py input/winmail.dat
The second argument is optional, it is the path where the resulting ICS file should be written. If this is omitted, the script will write to a file called invite.ics in the current directory.
python tnef2ics.py input/winmail.dat output/invite.ics
On macOS, it is also possible to use Automator to create a small app which can be used to run this script by using the "Open with" contextual menu on winmail.dat files, or by drag and dropping such files onto it.
To do this, create a new workflow in Automator and choose to save it as an Application. Find the Run Shell Script action, select "Pass input as arguments" and write the following as the code to be run:
for f in "$@"
do
/path/to/python /path/to/tnef2ics.py "$f" /tmp/invite.ics
if [ $? -eq 0 ]; then
open /tmp/invite.ics
fi
done
The if statement checks if the script exited without error before attempting to open the ICS file. Otherwise you risk opening an older ICS file without realizing.
Be sure to replace the placeholder text with your actual paths to the Python binary and the tnef2ics.py script.
Save the application thus created. You will probably need to grant Full Disk Access to it in order to work. Do so at your own risk:
- 2022-06-08 - Fixed timezone issue
- 2022-06-07 - Added instructions to run it as an Automator app
- 2022-06-07 - First version of the script. It successfully parses simple calendar invitations. All other information is ignored.