WARNING! My suggestions are addressed only to very experienced Nerds and I am not responsible for damage, data loss, etc.
Premise
The great Steve Jobs would never have allowed the Developers to create an ugly and above all interface adverse of ergonomics and annoying for Mac Users. From several versions of macOS, instead, Mail.app's Preferences panels continue to be ugly and prevent easy use for those who, like me, have to manage multiple Accounts, both personal and those of family members who have difficult with the computer.
The Mail.app's Preferences Accounts panel, for example, truncates the custom names in the Accounts box-list and oblige to scroll through all the names to find the account we are interested in modifying.
In fact, if a Mac User have many mailboxes (such as abcdefghij@abcde.com, abcdefghij@abcde.legal.com, abcdefghijfghailmnopq@gmail.com, etc.) it is very important and useful to insert the entire e-mail address name in the space for the customized names, instead of the simple name as Apple forces you to do (eg.: GMail, iCloud, Yahoo or another truncated name...)
Even the Signature panel is ugly! And moreover, moving from Font and Colors panel to Account or Signature panel, these panels will appear very narrow vertically and therefore truncate the names list also vertically!
Possible solution
It is shameful that an elderly doctor as I am can make a change with success that, instead, Apple should expect from its Developers, if he respected the users as Steve respected them.
The change that I suggest concerns the version of Mail.app 13.4 of macOS Catalina 10.15.5 and 10.15.6, but I have seen that even in Big Sur Apple has still left these ugly Mail.app Preferences.
I am attaching below some clips with the results after the changes I made to some .NIB files in the Basic.lproj folder, thanks to which I obtained that the Accounts and Signatures panels hare more elegant and ergonomic.
Note
Since the latest versions of Xcode no longer allow you to open NIB files to use Interface Builder and to make the Preferences panels resizable, as was at the time of Steve Jobs, I had to decompress the .NIB files (they turn into folders) and edit one of the contained files that are XML files. Obviously I struggled a lot to find the values to be changed...
For those of you who are capable, I indicate what I have modified in a copy of Mail.app that can be copied to the Desktop.
After testing the changes, those of you who are capable can restart the Mac in Recovery mode and through the Terminal can disable the system lock (csrutil disable) and replace the copy of Mail.app in /Applications/ folder.
I don't know if the rules of the MacRumors forum allow me to insert in this post the Base.lproj folder containing the NIB files I modified or if this violates the forum rules and also the Apple's rights and can give some legal problems... Kindly let me know, thanks
In any case, it is sad that with little effort I was able to beautify and make the Mail.app Preferences more functional, even if I am an elderly Doctor, I am not a developer, I am only a computer enthusiast and the last versions of Xcode no longer allow to use Interface Builder with the NIB files, while instead the Apple Engineers demonstrate that they don't want (or don't know?...) how to allow the perfect resizing of box-lists or panels (horizontally and vertically, even manually), like all parts of the macOS GUI, like all System in the world and even in the defective Windows... Using Xcode's Interface Builder, it would be very simple and it would take half an hour to do better than I did and would get better results...
How To
I used BBEdit to edit the XML file keyedobjects.nib which is contained in the decompiled .NIB files.
After being decompiled, in fact, NIBs become like folders with three XML files inside them.
You can search the Internet for how to decompile NIB files. (Eg.: you can see -> here the suggestion to use NIBDecompiler. Download and copy the NibDecompiler.workflow file to ~/Library/Services. Now there'll be a NibCompiler context menu entry on files which we can use to make the .NIB files editable.)
THE NUMBERS AT LEFT CORRESPOND TO THE BBEDIT LINES TO BE MODIFIED
The values in bold are my modified values
Fonts and Colors Preferences panel
Path: Mail.app/Contents/Resources/Base.lproj/FontsAndColorsPreferences.nib/keyedobjects.nib
455 <real>400</real>
456 <key>NSConstantV2</key>
457 <real>400</real>
4345 <real>550</real>
4346 <key>NSConstantV2</key>
4347 <real>550</real>
5413 <real>550</real>
5414 <key>NSConstantV2</key>
5415 <real>550</real>
6154 <real>550</real>
6155 <key>NSConstantV2</key>
6156 <real>550</real>
7265 <real>274</real>
7266 <key>NSConstantV2</key>
7267 <real>274</real>
Mail Accounts Preferences panel
Path: Mail.app/Contents/Resources/Base.lproj/PreferencesWindow.storyboardc/Bzm-EZ-vna-view-o72-6L-ZUG.nib/keyedobjects.nib
569 <real>210</real>
Mail Signature panel
Path: Mail.app/Contents/Resources/Base.lproj/SignaturePreferences.nib/keyedobjects.nib
1537 <string>{270, 400}</string>
Below some panels after my changes:
Premise
The great Steve Jobs would never have allowed the Developers to create an ugly and above all interface adverse of ergonomics and annoying for Mac Users. From several versions of macOS, instead, Mail.app's Preferences panels continue to be ugly and prevent easy use for those who, like me, have to manage multiple Accounts, both personal and those of family members who have difficult with the computer.
The Mail.app's Preferences Accounts panel, for example, truncates the custom names in the Accounts box-list and oblige to scroll through all the names to find the account we are interested in modifying.
In fact, if a Mac User have many mailboxes (such as abcdefghij@abcde.com, abcdefghij@abcde.legal.com, abcdefghijfghailmnopq@gmail.com, etc.) it is very important and useful to insert the entire e-mail address name in the space for the customized names, instead of the simple name as Apple forces you to do
(eg.: GMail, iCloud, Yahoo or another truncated name...)Even the Signature panel is ugly! And moreover, moving from Font and Colors panel to Account or Signature panel, these panels will appear very narrow vertically and therefore truncate the names list also vertically!
Possible solution
It is shameful that an elderly doctor as I am can make a change with success that, instead, Apple should expect from its Developers, if he respected the users as Steve respected them.
The change that I suggest concerns the version of Mail.app 13.4 of macOS Catalina 10.15.5 and 10.15.6, but I have seen that even in Big Sur Apple has still left these ugly Mail.app Preferences.
I am attaching below some clips with the results after the changes I made to some .NIB files in the Basic.lproj folder, thanks to which I obtained that the Accounts and Signatures panels hare more elegant and ergonomic.
Note
Since the latest versions of Xcode no longer allow you to open NIB files to use Interface Builder and to make the Preferences panels resizable, as was at the time of Steve Jobs, I had to decompress the .NIB files (they turn into folders) and edit one of the contained files that are XML files. Obviously I struggled a lot to find the values to be changed...
For those of you who are capable, I indicate what I have modified in a copy of Mail.app that can be copied to the Desktop.
After testing the changes, those of you who are capable can restart the Mac in Recovery mode and through the Terminal can disable the system lock (csrutil disable) and replace the copy of Mail.app in /Applications/ folder.
I don't know if the rules of the MacRumors forum allow me to insert in this post the Base.lproj folder containing the NIB files I modified or if this violates the forum rules and also the Apple's rights and can give some legal problems... Kindly let me know, thanks
In any case, it is sad
that with little effort I was able to beautify and make the Mail.app Preferences more functional, even if I am an elderly Doctor, I am not a developer, I am only a computer enthusiast and the last versions of Xcode no longer allow to use Interface Builder with the NIB files, while instead the Apple Engineers demonstrate that they don't want (or don't know?...) how to allow the perfect resizing of box-lists or panels (horizontally and vertically, even manually), like all parts of the macOS GUI, like all System in the world and even in the defective Windows... Using Xcode's Interface Builder, it would be very simple and it would take half an hour to do better than I did and would get better results...How To
I used BBEdit to edit the XML file keyedobjects.nib which is contained in the decompiled .NIB files.
After being decompiled, in fact, NIBs become like folders with three XML files inside them.
You can search the Internet for how to decompile NIB files. (Eg.: you can see -> here the suggestion to use NIBDecompiler. Download and copy the NibDecompiler.workflow file to ~/Library/Services. Now there'll be a NibCompiler context menu entry on files which we can use to make the .NIB files editable.)
THE NUMBERS AT LEFT CORRESPOND TO THE BBEDIT LINES TO BE MODIFIED
The values in bold are my modified values
Fonts and Colors Preferences panel
Path: Mail.app/Contents/Resources/Base.lproj/FontsAndColorsPreferences.nib/keyedobjects.nib
455 <real>400</real>
456 <key>NSConstantV2</key>
457 <real>400</real>
4345 <real>550</real>
4346 <key>NSConstantV2</key>
4347 <real>550</real>
5413 <real>550</real>
5414 <key>NSConstantV2</key>
5415 <real>550</real>
6154 <real>550</real>
6155 <key>NSConstantV2</key>
6156 <real>550</real>
7265 <real>274</real>
7266 <key>NSConstantV2</key>
7267 <real>274</real>
Mail Accounts Preferences panel
Path: Mail.app/Contents/Resources/Base.lproj/PreferencesWindow.storyboardc/Bzm-EZ-vna-view-o72-6L-ZUG.nib/keyedobjects.nib
569 <real>210</real>
Mail Signature panel
Path: Mail.app/Contents/Resources/Base.lproj/SignaturePreferences.nib/keyedobjects.nib
1537 <string>{270, 400}</string>
Below some panels after my changes:
