have Frontier fixed their system? it has been broken for weeks as far as I know.

To diagnose problems with Thunderbird, try the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Hi Matt, I tried your suggestions. First with add-ons disabled in Thunderbird, then with Win 8.1 in safe mode with networking. No joy there so spent 2+ hours with Frontier. We used various approaches including successfully sending the attachments from the Frontier mail site. We finally set up a Frontier account on my TBird from which I can send multiple attachments. I can also send multiple attachments from mail.spro.net but not from my spro.net account in TBird. So for now I think it's another call to SPro . Thanks for your advice.

Do you have any idea what is different between the working and non working?

How old is your modem?

Hi Matt, I did everything you suggested and after you asked about my modem, I decided to check out my LinkSys router first. I received a non-descript Windows error which unfortunately I can't remember now. Somewhere in the process I came upon "DNS changes". After reading about third party DNS servers I decided to change my DNS setting from automatic to 8.8.8.8 and 8.8.4.4 Another reboot of the computer and now Thunderbird e-mails with several attachments transmit successfully !!! I hope that is useful to someone but I can't explain why it worked. BTW Frontier spent ~2.5 hours helping including using LetMeIn to review my settings. I could send attachments with a Frontier account on TBird. Thanks for your help.