Being your Mac’s default e-mail reader, Mail is likely the program you use to read messages on your various internet accounts (Google, Yahoo, iCloud, among other).
However, for some who use Mail, after upgrading OS X you might find a problem where the text cursor behaves sporadically, and will move backward a few characters every now and then as you enter text, interrupting your ability to type continuously. In addition, you might see garbled output, either as you type or as you read messages sent to you.
If this problem happens to you, then you might be experiencing an issue with custom text encoding settings for Mail, where the program is holding onto an old setting for changing the text encoding in prior versions of Mail, and wrongly applying it to the latest version that you are using.The latest versions of OS X Mail should encode all text in UTF-8; however, UTF only became popular starting in 2007-2008, and since then has risen to become the dominant text encoding. During this transition, Mail included options to select the desired text encoding for messages, which included UTF-8, ASCII, and others. If you changed this setting in prior OS X versions, and then upgraded your OS over the years, then your Mail preferences might still retain the encoding settings, which could be interfering with Mail’s current handling of your text, and resulting in odd problems. This issue might be especially true if you had applied custom encoding settings via the Terminal in the past.
To fix this problem, you will need to remove the encoding settings from Mail’s preferences. While in most cases removing the preferences file itself and then having the program rebuild it from scratch is the easiest approach for fixing such problems, in Mail there are a number of customizations and account information that will be lost if you do this, so first attempt to fix the problem by running the following command in the Terminal utility:defaults delete com.apple.mail NSPreferredMailCharset
Alternatively, you can run the following command to have this setting be the default “UTF-8” encoding:defaults write com.apple.mail NSPreferredMailCharset -string "UTF-8"
With this done, quit and re-launch Mail, and hopefully the problem will be gone. If not, then you might need to remove your Mail’s preferences file to address this issue. The file is called “com.apple.mail.plist” and is located in the username > Library > Containers > com.apple.mail > Data > Library > Preferences folder. Move this file to your Desktop (to retain a copy of it so you can restore it if needed), followed by re-launching Mail. You will have to reconfigure your mail accounts, smart mailboxes, and program organization and settings after doing this, but hopefully it will fix the issue. If not, you can restore the file on your Desktop and then be back up and running in Mail.