Sending e mails with an attachment from Thunderbird to Outlook 365 , each time the name of the attachment is long, the attachment name is displayed by Outlook 365 with a modified name. The modified name, as displayed by Outlook 365, starts with the initial digits of original name as sent by Thunderbird, followed by a long sequence of alphanumeric characters (e.g. %65%6E%5F%53%63%68%61%64%65%6E%5F%4C%65%74%69%7A%69%61%20%52%6F%74%61%20%28%33%29%2E%70%64%66 ) The above happens with PDF attachments . The PDF contents are not corrupted, but the file name is changed. (In the case of a .docx attachment with a the same long name, in addition to displaying the wrong name, the attachment itself became in Outlook 365 a .dat file which could not be open). Is any configuration change I should introduce in Thunderbird to overcome this incompatibility? Different PCs are showing the same issue. OS: Windows 2004 with the Sept 2020 Cumulative Update. Office 365 updated with the Sep 2020 update. Thunderbird 72.2.2 - 32 bit with standard settings and Unicod (UTF-8) - German Language The problem might have been preexisting, I just noted it now. (I also wrote about the issue in a Microsoft forum, it has been initially suggested to repair Outlook 365 but this did not help, other investigation steps may still come)

Only for information: I had raised the issue in the Microsoft Community, some time ago. I then received a reply, stating among others: "The Outlook Team has developed a fix for this issue that will go to Current Channel in late November". In summary, it looks that Microsoft are taking care of this compatibility issue with Thunderbird

Just a guess, but Microsoft support of RFC 2231 has been non existent up until now.

Note that replying to your own question actually significantly reduces your chance of someone responding. A reply, any reply removes your request from the "new" category.

Thanks for the tip concerning the reply to myself. Indeed, I did it on purpose, because I had raised the question in September and never got a reply, until your reply! Concerning Microsoft support to RFC 2231, I would not be so negative, I raised the question and it was answered very rapidly. Other users joined having the same problem, then we got the reply indicating a future update of Outlook 365 to address the issue. I noted that beta version including this correction was already existing in summer, for what I understood. Hopefully Outlook will release the update in the stable channel in the coming future, let's see. In any case, Outlook for Android, Outlook web and Windows Mail do not have this compatibility issue with Thunderbird when the name of attachments is long (and also containing special characters like ä,ü etc). The attachment there received are also correctly displayed.