Okay, I originally thought this bug only affected the Outlook Mail app on Windows 10 Mobile.
As I've continued to research this and speak to other users I've concluded that the bug has nothing to do with Outlook at all, but rather the touch keyboard in Windows 10 (both PC and Mobile).
With that said, this is posted in the completely wrong forum. I will keep the post here if it's okay with moderators though, since my Feedback Hub post is linking to this URL.
What I have surmised so far is this: the bug affects Scandinavian touch keyboards (Swedish, Norwegian, Danish, Finnish) in conjunction with Word Mobile, Word Online and the Outlook/Windows 10 Mail UWP. (It might affect more keyboards. Don't have the time to test them all.
Have however tried with French, Italian, English US and Englisk UK, and none of those are affected.)
The @ symbol on these keyboards has somehow stopped working as intended. (I.e. it doesn't print out a @.)
When using your touch keyboard on either a Windows 10 Mobile device or a Windows 10 pc/tablet, the @ sends a command to change the style of the text from "Normal" to "Heading 2".
My original post is still below. What happens in the video at the bottom is the same result I've been able to reproduce when trying the other scandinavian languages, and when testing the touch keyboard on my Surface Pro with Word Mobile, Word Online and Windows 10 Mail.
Original post with original edits:
Edit: I've tested with Danish keyboard too. The bug is present there too. So this is not only affecting the Swedish keyboard.
Edit 2: This bug is affecting not only Outlook, but Word Mobile as well.
Edit 3: After more research I've found that this bug not only affects Windows 10 Mobile, but also the PC version. I've reproduced the bug in both Windows 10 Mail and Word Mobile on my Surface Pro tablet. The bug only occurs when using the on screen keyboard (touch keyboard), not while using a physical keyboard.
While using the Swedish keyboard for writing an email in the Outlook Mail app on Windows 10 Mobile the "at-symbol" doesn't work as intended. Instead of outputting a simple "@" it changes the formatting of the text in the mail to "Header 2".
English keyboard works fine.
Example of ordinary mail:
<html xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"><head><meta http-equiv=Content-Type content="text/html; charset=utf-8"><meta name=Generator content="Microsoft Word 15 (filtered medium)"><style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
.MsoChpDefault
{mso-style-type:export-only;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:70.85pt 70.85pt 70.85pt 70.85pt;}
div.WordSection1
{page:WordSection1;}
--></style></head><body lang=SV><div class=WordSection1><p class=MsoNormal>This is the body of the email when not writing an ”at symbol”</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Sent from my Windows 10 phone</p><p class=MsoNormal><o:p> </o:p></p></div></body></html>
Example of mail sent after trying to use the @-symbol:
<html xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"><head><meta http-equiv=Content-Type content="text/html; charset=utf-8"><meta name=Generator content="Microsoft Word 15 (filtered medium)"><style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:"Calibri Light";
panose-1:2 15 3 2 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
h2
{mso-style-priority:9;
mso-style-link:"Heading 2 Char";
margin-top:2.0pt;
margin-right:0cm;
margin-bottom:0cm;
margin-left:0cm;
margin-bottom:.0001pt;
page-break-after:avoid;
font-size:13.0pt;
font-family:"Calibri Light",sans-serif;
color:#2F5496;
font-weight:normal;}
span.Heading2Char
{mso-style-name:"Heading 2 Char";
mso-style-priority:9;
mso-style-link:"Heading 2";
font-family:"Calibri Light",sans-serif;
color:#2F5496;}
.MsoChpDefault
{mso-style-type:export-only;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:70.85pt 70.85pt 70.85pt 70.85pt;}
div.WordSection1
{page:WordSection1;}
--></style></head><body lang=SV><div class=WordSection1><h2>This is the body of the message and what happens with it when trying to write an ”at symbol” <span style='font-size:11.0pt'><o:p></o:p></span></h2><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Sent from my Windows 10 phone</p><p class=MsoNormal><o:p> </o:p></p></div></body></html>
The very simple work around is to just change keyboard to English whenever you need to write a @.
Here's a video of the bug in action.
