Calibre can't "Send-to-Kindle E-Mail"

It appears that Calibre can no longer "Send-to-Kindle E-Mail". It has worked correctly in the past using Calibre v2.61 and configured to use my ISP's smtp server.

It appears to have stopped working when I upgraded to v2.62.

Hoping to correct the problem I am now using Calibre v2.63. A test email sent from Calibre works correctly.

Calibre can successfully send email to my email account. My email account can successfully "Send-to-Kindle E-Mail", and Calibre appears to send directly to my Kindle account, but the ebook never shows up in my list of "Personal Documents" and I get no error message either from Calibre or Amazon.

As a work around I send an ebook from Calibre to my email and then forward it to my "Send-to-Kindle E-Mail", after which the ebook shows up in my list of "Personal Documents" under my Amazon account.

I have confirmed that the email address Calibre is using is in my "Approved Personal Document E-mail List".

So to summarize: Calibre can send me an ebook, I can forward the ebook to Amazon, but Calibre can't send directly to Amazon. The last time it worked I was using Calibre v2.61.

Any help would be appreciated.

Thanks, MyKill.