Ask your IT department what the server supports.

• no encryption? port?
• STARTTLS? port?
• SSL/TLS? port?

What anti-virus software are you using, and does it intercept your secure communication with the server?

SMTP is on port 587

Connection security: STARTTLS Auth Method: NTLM

Please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information. Press the Copy text to clipboard button and paste the information into your reply.

Copy text below:

 Application Basics

   Name: Thunderbird
   Version: 45.1.0
   User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:45.0) Gecko/20100101 Thunderbird/45.1.0
   Profile Folder: Show in Finder
   Application Build ID: 20160509110721
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account6:
     INCOMING: account6, , (imap) mail.cphart.co.uk:143, alwaysSTARTTLS, NTLM
     OUTGOING: , mail.cphart.co.uk:587, alwaysSTARTTLS, NTLM, true

 Crash Reports

 Extensions
   ExQuilla for Microsoft Exchange, 45.0.1161, true, exquilla@mesquilla.com
   Lightning, 4.7.1, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   Theme Font & Size Changer, 46.5, false, {f69e22c7-bc50-414a-9269-0f5c344cd94c}

 Important Modified Preferences

   Name: Value

     browser.cache.disk.capacity: 358400
     browser.cache.disk.filesystem_reported: 1
     browser.cache.disk.smart_size_cached_value: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     extensions.lastAppVersion: 45.1.0
     font.internaluseonly.changed: true
     font.size.fixed.x-western: 16
     general.autoScroll: false
     gfx.blacklist.direct2d: 3
     gfx.blacklist.layers.direct3d9: 3
     gfx.blacklist.webgl.msaa: 4
     mail.openMessageBehavior.version: 1
     mail.spotlight.firstRunDone: true
     mailnews.database.global.datastore.id: 623ad97a-7f05-fe4b-8eed-b6401a65e43
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     mailnews.database.global.views.global.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     network.auth.force-generic-ntlm-v1: true
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1464193480
     places.history.expiration.transient_current_max_pages: 104858
     plugin.importedState: true
     print.macosx.pagesetup-2: PD94bWwgdmVyc2lvbj0iMS4wIiBlbmNvZGluZz0iVVRGLTgiPz4KPCFET0NUWVBFIHBsaXN0IFBVQkxJQyAiLS8vQXBwbGUvL0RURCBQTElTVCAxLjAvL0VO…
     security.disable_button.openCertManager: false

 Graphics

     Vendor ID: 0x1002
     Device ID: 0x6740
     WebGL Renderer: ATI Technologies Inc. -- ATI Radeon HD 6770M OpenGL Engine -- 2.1 ATI-1.6.36
     GPU Accelerated Windows: 0

     AzureCanvasBackend: skia
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: none
     AzureContentBackend: quartz

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.12
     4.12

     NSS
     3.21.1 Basic ECC
     3.21.1 Basic ECC

     NSS Util
     3.21.1
     3.21.1

     NSS SSL
     3.21.1 Basic ECC
     3.21.1 Basic ECC

     NSS S/MIME
     3.21.1 Basic ECC
     3.21.1 Basic ECC

OUTGOING: , mail.cphart.co.uk:587, alwaysSTARTTLS, NTLM, true

As per your Troubleshooting Information your server settings for SMTP are exactly as requested by your IT folks.

An error occurred while sending mail: Unable to establish a secure link with Outgoing server (SMTP) mail.cphart.co.uk using STARTTLS since it doesn't advertise that feature.

The error message suggests the problem is on the server. So I'm afraid you'll need to go back to your IT folks.

Did this ever work before? Were there any changes recently?

It's been working fine for the last two weeks and my IT dept are saying nothing has been changed. I can even telnet on to the mail server from my mac, so my IT dept are saying its related to thunderbird.

You can run the following command from a terminal. Please post the result.

openssl s_client -connect mail.cphart.co.uk:587 -starttls smtp </dev/null </p>

I get this response:

CONNECTED(00000003) didn't found starttls in server response, try anyway...

I suppose that wasn't all you got?

Thats all terminal came back with.

Sorry very slow to respond but i just got this:

2681:error:140770FC:SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol:/SourceCache/OpenSSL098/OpenSSL098-35.1/src/ssl/s23_clnt.c:607:

Take that response and show it to your IT folks. There is no proper TLS handshake, and the encrypted connection is never established.

Ok thanks for your email, I've forwarded that info on to my IT dept. Doesn't help I'm the only one with a Mac in the company.

Your Troubleshooting Information shows you do have the 'ExQuilla for Microsoft Exchange' add-on installed. Why do you need it and what are you doing with it?

My IT dept tried to use that plugin when I had some problems a few weeks back, I don;t use it.

Then you should either disable or remove it via the Add-ons Manager.