Mailcow appointment -> @outlook.de: "not supported calendar message.ics"

ttfob

Hello,
we’ve been using mailcow for a while now. We’re using the newest update “2025-07”.
Today we created an appointment with an external xxx@outlook.de account.

The outlook.de account isn’t able to read/accept the appointment. It also changes the “invite.ics” attachment into “not supported calendar message.ics”. There is no further information why that happened.

I created a free, fresh outlook.de account and it’s the same behavior.
It works fine with other groupware solutions like gmail.com.

I checked the invite.ics file with different ics validators: passed on 3 different validators. So it looks like it’s created using the rfc standards.

Has anyone observed similar behavior?
Is there some kind of solution?

Thanks in advance 😉

esackbauer

How did you create the appointment? In SOGo? Thunderbird? Outlook?

ttfob

esackbauer
We created the appointment using SOGo.

I just tested it with thunderbird: it works. So it’s a SOGo problem.

esackbauer

Then you only can try if it is already listed here in their bugtracker:
https://bugs.sogo.nu/my_view_page.php

If not then someone need to raise this issue on their mailing list.

ttfob

Alright, I found the ticket / problem.
"If Sogo user create event invitation trough webmail with reminder inside and send it to a Microsoft/Exchange/Outlook user, the Microsoft/Exchange/Outlook receives the invitation with “not supported calendar message.ics” attachment to it and is not able to add it to his calendar."
https://bugs.sogo.nu/view.php?id=4801

I tested it with an appointment without any reminder and it worked.
Thanks for pointing me to SOGo list. I am gonna report it.

ttfob

Little update:
Looks like it is already fixed in SOGo v5.12.1 released.
So we have to wait until mailcow updates its SOGo component.

https://www.sogo.nu/news/2025/sogo-v5121-released.html
“calendar: put valarm at the end of vevent block to satisfy outlook.office.com which is too lazy to follow the spec (148fab9)”