You can try setting it on registry so your custom profile folder will be used even if Opera isn't started with the customised short-cut.

In this case, I think you'd have to edit the commands in these keys:

HKEY_CURRENT_USER\SOFTWARE\Classes\http\shell\open\command
HKEY_CURRENT_USER\SOFTWARE\Classes\https\shell\open\command

Maybe the HKEY_LOCAL_MACHINE ones too/instead or:

HKEY_CLASSES_ROOT\http\shell\open\command
HKEY_CLASSES_ROOT\https\shell\open\command

Your changes will be overwritten though the default browser for those is ever modified.

You might be able to edit:

HKEY_CLASSES_ROOT\OperaDeveloper\shell\open\command

for example instead, so that when Opera is set as the default browser again, all the commands in the other keys are updated too. For stable Opera, that might be:

HKEY_CLASSES_ROOT\Opera.Protocol\shell\open\command

There are others besides Opera.Protocol like Opera.HTML for .html files. Always look at the command though first. Some of the keys might be for old Opera 12.

Basically command-line options don't help much except for shortcuts unless you change them in the registry too. If you can specify the user data path in a file that opera.exe can read (maybe look at chrome configuration docs), that'd be better.

Is there another option in opera changing the user-dir?

Nope. You can try setting it on registry so your custom profile folder will be used even if Opera isn't started with the customised short-cut.

I open opera with the option:

--user-data-dir=D:\Opera_settings

And when i click on a link in an e-mail-program the "standard" folder for settings is opened.

So for opera this is a new instance.

Is there another option in opera changing the user-dir? I find nothing in the windows settings for standard-apps or in Thunderbird to change this?

Any idea?

I tried several things:

1. i switched in Windows 10 the standard app options to Firefox or Edge and everything work normal. Everytime i click on a link in an email-programm only one instance will be opened - if one instance of firefox or edge is acuall open then the link will be opened in the open instance in another tab --> everything works correct.

2. Now i switch back on the standard app options in Windows to opera - i have the problem: one instance of opera is open and i click a link in Thunderbird, Outlook 2016 or Windows Mail - everytime, it doesn´t matter which e-mail-programm, a new instance of opera will be opened. When i click now on another link in any e-mail programm then the link will be opened in the just opened instance in another tab. When i click on more links in any e-mail programm then the links are opened in another tab in the same instance of opera.

3. There is no instance of Opera open. When i click on a link in any e-mail-programm, then a new instance of opera will be opened and the link is opened in a tab --> everythin correct. Nowi click on another link in any e-mail-programm the link will be opened in the same instance of opera in another tab --> everythin works correct.

4. I tried this on another computer. Here everythin works correct. No problem with opera.

I have only the problem, that when opera is aöready open - a new link will be opened.

I believe that has nothing to do with any e-mail-program. An option in Windows? Or Opera? But what option?

Have someone any idea? What option could this be?

I remember to have used, a very long time ago, a program with such setting.