Hi majwaj, This will happen to me as well when the Thunderbird the mail client has been open for a while and a password is required to sent the mail. Entering the password usually does this for me. Try this: Unable to authenticate to SMTP server and also check the account profile.

Checked with provider that not blacklisted and can use through ISP webmail with no probs.

This seems to refer to Thunderbird 2 & 3. I am on 24.3.0.

https://support.mozilla.org/en-US/kb/cannot-send-messages

Going round in circles here. My normal setting is "password, transmitted insecurely". I can send if I set it to "No authentication" but obviously do not want to do that permanently. There are 3 other options and I will check with my SP to see if I can use any of them. I can still use the email through the SP's webmail but it is rubbish compared to Thunderbird.

Hey majwaj,

In your sever set up is the sntp port correct: https://support.arvixe.com/index.php?.../do-you-support-secure-smtp-pop-and-imap "The secure POP3 port is 995......... not 993"

majwaj, can you post your Troubleshooting Information?

AppMenu button - Help - Troubleshooting Information

Application Basics

   Name: Thunderbird
   Version: 24.3.0
   User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20140131124303
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Crash Reports: about:crashes
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (imap) mail.tesco.net:143, plain, passwordCleartext
     OUTGOING: smtp.tesco.net:587, plain, passwordCleartext, true

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

 Extensions
   British English Dictionary, 1.19.1, true, en-GB@dictionaries.addons.mozilla.org
   Test Pilot for Thunderbird, 1.3.9, true, tbtestpilot@labs.mozilla.com

 Important Modified Preferences

   Name: Value

     browser.cache.disk.capacity: 358400
     browser.cache.disk.smart_size.first_run: false
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 Graphics

     Adapter Description: Intel(R) G41 Express Chipset
     Vendor ID: 0x8086
     Device ID: 0x2e32
     Adapter RAM: Unknown
     Adapter Drivers: igdumd64 igd10umd64 igdumdx32 igd10umd32
     Driver Version: 8.15.10.2104
     Driver Date: 3-31-2010
     Direct2D Enabled: Blocked for your graphics driver version. Try updating your graphics driver to version 8.1500.1000.2202 or newer.
     DirectWrite Enabled: false (6.2.9200.16571)
     ClearType Parameters: Gamma: 2200 Pixel Structure: RGB ClearType Level: 100 Enhanced Contrast: 100
     WebGL Renderer: false
     GPU Accelerated Windows: 0

     AzureCanvasBackend: skia
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: none

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.10.2
     4.10.2

     NSS
     3.15.4 Basic ECC
     3.15.4 Basic ECC

     NSS Util
     3.15.4
     3.15.4

     NSS SSL
     3.15.4 Basic ECC
     3.15.4 Basic ECC

     NSS S/MIME
     3.15.4 Basic ECC
     3.15.4 Basic ECC

AppMenu button - Options - Account Settings

In the Account Settings window select 'Outgoing Server (SMTP) at the bottom of the left pane.

Select your SMTP server - Edit

Select 'None' from the 'Connection Security' drop-down menu. This should change the port to 25.

Try again.

Since Tesco do not seem to support SSL/TLS I'd look for another email provider.

Already on "None" port stays at 587 so no dice!

Then manually set the port to 25.

Asks for password then does not send so no change

Ok, I am seriously confused by this thread.

Tesco, like many ISPS may well not require any authentication to send mails. There is nothing requiring it in the RFCs other options available to the server owner to only allow authenticated people to send mail on their server is to insist you connect to the server from within their network. another perfectly valid option is to require you to "get mail" immediately before or within a time frame of sending.

So the issue is. Can you find a setting that allows you to send mail? One of the replies above indicates it works but your not happy with the level of security. Connection security or lack there of is set by the server owner (TESCO). In this case given that they are a grocer selling broadband. They will have the least security they can legally get away with because security increases complexity and support cost.

Note also that port 25 may work, but is primarily for server to server communications. The "standard" port for non encrypted connections is 587, although there are some major players who think that the new standard does not apply to them.

Original settings attached. Tried the suggestions; the first gave a message basically "unknown server"; plain mail.tesco.net gives the same result as the original and keeps trying to connect to mail.etc; (smtp)mail. etc is not a valid server name.

Tesco have eventually come back to say they are having problems without going into specifics. Time to change provider if it wasn't for the hassle.

ok, mail.tesco.net is correct per tesco see http://www.tesco.net/help_pages/questions/server_settings.html however you have created an IMAP account and Tesco only have POP servers.

So remove the account entirely. There is an account actions button under the list box in Tools menu (Alt+T) > account settings and add it again as type POP and you will probably stop having receive errors

a) not having receive errors b) Tesco allows POP3, IMAP or HTTP anyway Tesco say they have a problem and there are weird things happening with the synchronisation between the Thunderbird and webmail versions ofmy wife's tesco.net email which is thro' the same router. So I am going to hang fire until I get a clear reply from Tesco