Look at addons such as Exquilla or Davmail. Or find a better email provider.

Well, Zenos, The email I use is via Florida State University - a fairly large institution. Sorry you think it's not a good provider. I tend to think that Thunderbird is not such a good client since it can't handle exchange protocols.

From FSU documentation http://its.fsu.edu/Email/Exchange/ClientSetup

under IMAP Settings

IMPORTANT: You will not be able to send email unless 
connected to an FSU network when using these 
settings. It is recommended that you use Exchange 
settings, if possible.


Email Clients:

Microsoft Outlook 2010/2013
Microsoft Outlook 2007
Mozilla Thunderbird

I read that to mean that they are not discontinuing SMTP, but are not allowing its use if you are not logged into their network.

If you still want to use TB with exchange protocols -

http://www.techrepublic.com/blog/smb-technologist/connect-the-thunderbird-email-client-to-your-exchange-server/

"Exchange" is shorthand for Microsoft Exchange Server. It uses closed, proprietary protocols to communicate preferentially with Microsoft email clients. That doesn't make Thunderbird a bad email client, when it's Microsoft who choose to create their own peculiar systems, and not to publish details of same.

Note that the link provided by gild also recommends one of the add-ons that I mentioned, ExQuilla.

However, if the operators at FSU choose to enable standards-compliant protocols, namely IMAP, POP and SMTP then any standards-compliant email client would be able to work with Exchange. As gild says. the restriction seems to be on your location and connectivity, and not necessarily about withdrawal of the SMTP protocol.

I suffer an Exchange server here at my workplace. Fortunately it has IMAP and SMTP enabled, so I am free to use Thunderbird with it. But I have used DavMail successfully with it in the past.

As regards a "better" email provider, I would, where the choice existed, take care to avoid any email provider who offered only Exchange and obliged me to use Outlook. Some of us choose not to pay Microsoft for their wares, and don't use Windows, so running Outlook is essentially a non-starter.

FSU's stated reason to drop SMTP for not trusted IPs is to save the students/staff/faculty from phishing. Bull. It is a result of embarqmail blocking 2 IP addresses in use by exchange to send email out due to those addresses being "spammy". FSU, years ago, changed to Outlook to allow for group think/calendars, etc that Outlook allows. Personally I never use that stuff in the email client and would much prefer not to have to use Microsoft for email. I can still read messages with Thunderbird using IMAP but I can't send anything out using Thunderbird. I have to use the web page that FSU provides. (oddly, the built in email client on my Samsung S6 supports exchange and I can use it as per normal) Problem comes in because I have been using Thunderbird to store/archive email locally. I have lost the ability to then call them up and use them as a basis for a send email without a lot of cut/paste (I'm basically lazy) or to easily search the entire database. Exquilla requires a yearly fee to use. I haven't tried Davmail. ((note that I also have a gmail and an embarqmail accounts and both require a web login to be able to send email; embarqmail I can read but not send))

I guess what I really want is for Thunderbird to build in an option to use exchange protocols so I can continue to use Thunderbird. Else, I'm stuck using web based email (shudder)

gild, on September 1 the university tightened email. You can still use IMAP to receive mail but to use SMTP you must be on a trusted IP, whichever they are. Here on CenturyLink DSL I am not on a trusted IP. The new supported client list is: Email Clients:

   Microsoft Outlook 2010/2013
   Microsoft Outlook 2011(Mac)
   Microsoft Outlook 2007
   Entourage

Mobile Devices:

   Android
   iOS (iPad / iPhone)
   Blackberry Internet Service (BIS)*
   Windows Mobile 7
   Windows Mobile 6

Because you have a gmail account set up in Thunderbird, Google will allow you to send any of your messages using their SMTP server. Just set up the FSU account to use the Outgoing server listed for gmail.

TB 38.2 Win10-PC

Added Google does not care about where the message came from, or is stored on your machine. They want to know that you are a registered user.