Send to Kindle Not Working/Stopped Working Fix
Although you may be greeted by the ‘ Content will be uploaded to your kindle soon’, it may never arrive. If that’s the case, I suggest using Calibre (Free) to send out your kindle documents.

Will I still be able to Sync Progress, Highlights etc?

The short answer is yes. Calibre maintains the book format (.mobi, .azw., .prc) as well as keeping your embedded fonts and text align, etc. Note: The format must be .mobi or KF8, also known as hybrid files. AZW files are not supported to send via email but there’s a workaround. Convert your book to the .mobi format using Calibre and choose ‘both’ instead of old or new. This will output a .mobi file that acts as .azw, retaining format and publisher fonts, etc. when shared via email.

Your Kindle Account

First thing’s first. Let’s set up device synchronization as well as other settings to ensure smooth sailing. Login to your Amazon account and head to:

Account > Manage Your Content and Devices > Settings

Device Synchronization (Whispersync Settings)

Make sure this is enabled.

Send-to-Kindle E-Mail Settings

Here is a list of your personal kindle email addresses. You can use any of the ones listed there. I personally use the same one: mykindle@kindle.com

Approved Personal Document E-mail List

Here is where you can whitelist a particular email you own and plan to send from so it won’t be marked as spam. Personally, I use something along the lines of mykindle@gmail.com.

It is important to have this settings defined because we will be making use of them in Calibre later on.

Send to Kindle Service + Calibre

Launch Calibre. If it’s your first time, you will be taken to the Welcome wizard (which can be launched at any given time). If not, head over to preferences > sharing books by email.

Add email: Define your send to kindle email. In my case, mykindle@kidle.com and where it says: send email from: Type your personal whitelisted email address. In my case: mykindle@gmail.com. Typically, it’s not necessary to fill out the mail server information. You can test the service by clicking on ‘ Test email’. The output will be something like: Email successfully sent. You will receive an error from Amazon since you didn’t actually sent anything!

So there you have it. If Amazon is no longer letting you send out .mobi, .azw, etc. Try Calibre! There really is no reason not to use it. This is a free and robust software and while it can be a learning curve, it makes the reading experience much better. Calibre has other functionality not detailed here, such as plugins for hyphenation, multiple book format conversions, being able to extract highlights, etc.