This will happen with every new versions of Mail.app, that's something we have to live with when using plugins form Mail. There's however a good news: most of the time all you have to do is to run the installer of Universal Mailer again, it will magically re-enable the plugin for you!
There's however a catch here: if a major OS X update comes out (Mavericks is a good example) be sure to check this website for updates, there might be a new version that needs to be used with the updated Mail.app!
It largely depends on your personal tastes and also what you mean by 'readable', however I suggest to start with a Helvetica fonts with size 14, and I would specify using points instead of pixels.
This is not a problem of the plugin: the default font is set for all the text that hasn't other font specifications: if you set a specific font for a part of the body (signature included) it will retain your size, not the default one.
Also, be sure to use points and pixels consistently: if you use 15 points in your body, specifying 15 pixels in your signature might be the same on your screen but it certainly change with other screen resolutions! Be sure to double check your signature specification and configure Universal Mailer to do the same.
This plugin is not signed with an Apple Developer Certificate, and this is the reason you get this error. You can override this problem by right-clicking the installation file and select 'Open'. This will produce a warning also, but you can continue with the installation. Another method, which is not advised since it might expose to security risks, is to temporary (or permanently, if you wish to do so) disable this extra security check to install the plugin, here's how: go to System Preferences, select Security & Privacy and select 'Anywhere' in the 'Allow apps downloaded from'. This operation might require you to click on the yellow on lock on the bottom left of the window and enter your password. After you installed the plugin you can safely switch to your previous setting, too.
If you're done with the plugin, the easiest way to remove it is to use Mail Plugin Manager, a free software provided by MailTags.
First, check if you have the plugin installed: you can tell this by opening Mail.app's Preferences. If you see a '>>' symbol on the top right or the Universal Mailer logo, the plugin is there.
Be sure to avoid conflicts between Mail.app's plugins: due to the way Apple handles third parties plugin in Mail, two similar plugins might lead to one of them not working. There are no brilliant solutions here: just try one plugin at a time and keep only those you prefer.
If the plugin is not installed you can run Mail Plugin Manager, it will help you manage Mail.app's plugins.
If you still can't see the plugin, you can run a few commands from Terminal.app (you can find it under /Applications/Utilities) to ensure that everything is correctly set up. Some of those commands might ask you for a password: enter your login password followed by the enter key. Please note that there won't be any echo to the screen when you type the password and, if you never used 'sudo' before, the system will warn you about its usage: it's all ok, the commands listed here won't do anything harmful to your computer and you can safely enter the password!
> defaults write com.apple.mail EnableBundles -bool YES
> defaults write com.apple.mail BundleCompatibilityVersion -int 3
> sudo defaults write com.apple.mail EnableBundles -bool YES
> sudo defaults write com.apple.mail BundleCompatibilityVersion -int 3
This error might be shown when the installer failed to detect the version of Mail.app you have. To solve this problem you can run Mail Plugin Manager.
There are various reasons for this and here's a list of them, it is not complete nor in a particular order:
If you encounter a bug inside the plugin you should first go through all the FAQs (and since you are reading this I suppose you already did so), then try to identify the steps that caused it and submit a new issue. It is of much help to read the existing issues to avoid duplicates: the more duplicates the more the time to read them, the more the time to read them the lesser the time available to look at the code and solve it. Sounds simple and complicated at the same time.
Also, if you tag the issue with 'Bug' it speeds things up when reviewing open issues.
Think about it twice: are you sure this feature should be included inside UniversalMailer? If so, describe it by creating a new issue. It helps if you tag it as 'enhancement' and it also really helps if you explain it in simple terms, don't assume I'm smart :-)
Great! There are many ways to help here: you can either contribute by improving the code (cloning the project and filing a merge request is the way to go) or with anything you think it might help with the plugin such as improvements in the messages, improved graphics, help with the website, you name it...
If you however are not into programming, website design, etc. a donation is always appreciated!
If nothing here seems to help, you can send an email at universal.mailer.noware AT gmail.com, you'll get support as soon as possible!
It's as easy as a one man team, i.e. that's only one developer: me. This is to say you shouldn't expect frequent release or prompt responses: it largely depends on how busy I am at the moment. I promise I'll read all your emails and issues but you can experience a delay in the response from 1 hour to 1 month or something similar :-)