In order for us to better investigate this issue, we'll need some more information.

Could you please do the following:

1. Open the menu > Help > Troubleshooting Information, then click Copy text to Clipboard.
3. Open a reply to this post, and paste in your troubleshooting information

Next, we'll need you to provide answers to the following question:

• Do you use anti-virus and firewall software? What is the version?
• Who is the email provider?
• What is the exact error message?

I am guessing what you posted as the error message is an abridged version. But guesses can also be wrong.

Application Basics

   Name: Thunderbird
   Version: 68.7.0
   Build ID: 20200407160932

   Update Channel: release
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0
   OS: Windows_NT 10.0

   Launcher Process: Enabled
   Multiprocess Windows: 0/0
             Disabled
   Remote Processes: 0
   Enterprise Policies: Inactive
   Google Location Service Key: Missing
   Google Safebrowsing Key: Missing
   Mozilla Location Service Key: Missing
   Safe Mode: false

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) pop3.dodo.com.au:110, alwaysSTARTTLS, passwordCleartext
     OUTGOING: , smtp.dodo.com.au:587, alwaysSTARTTLS, passwordCleartext, true

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

   account3:
     INCOMING: account3, , (im) prpl-gtalk, plain, 0

   account4:
     INCOMING: account4, , (im) prpl-gtalk, plain, 0

 Crash Reports for the Last 3 Days

 Remote Processes

   Type: Count

 Extensions

     Name
     Version
     Enabled
     ID

       Amazon.co.uk
       1.1
       true
       amazon@search.mozilla.org

       Bing
       1.0
       true
       bing@search.mozilla.org

       DuckDuckGo
       1.0
       true
       ddg@search.mozilla.org

       Google
       1.0
       true
       google@search.mozilla.org

       ImportExportTools NG
       4.0.4
       true
       ImportExportToolsNG@cleidigh.kokkini.net

       Lightning
       68.7.0
       true
       {e2fda1a4-762b-4020-b5ad-a41df1933103}

       Twitter
       1.0
       true
       twitter@search.mozilla.org

       Wikipedia (en)
       1.0
       true
       wikipedia@search.mozilla.org

 Security Software

   Type: Name

     Antivirus:
     Antispyware:
     Firewall:

Kaspersky internet security dodo rollytaddeo2@dodo.com.au

this the actual error thats keeps coming up every time i send an email The mail server sent an incorrect greeting: smtp687.mel.oss-core.net Off-net-IP Detected. Regards rolly hi matt

turned off kaspersky antivirus  clicked send email and did not work

I don't think Dodo supports TLS on either the incoming or outgoing servers. Try setting the connection security to None and the authentication to password, transmitted insecurely. If port 587 for the outgoing doesn't work, try 25.

i change the connection sercurity to NONE and authentication to password, transmitted insecurely. and server port from 110 to 25 and DID NOT work. i am connected to dodo internet at home BUT i have taken the computer to my daughter house and they are connected with OPTUS

Ports 587 and 25 are for the outgoing server; leave the incoming at port 110. But the problem is that you're trying to send on a non-secure port when connected to a non-Dodo network, and that generally doesn't work. Until you return home, you should be able to send by accessing the account through webmail.

Thew problem i think is dodo only allow access to sending mail if you connect to the internet using their ISP services. I think that is what this cryptic message means.

: smtp687.mel.oss-core.net Off-net-IP Detected. 

I had my last experience with dodo some 2 decades ago when they had a cheap over subscribed dial up service that auto kicked you after 10 minutes. The cost of the dial up calls exceeded their plan costs. So I am not an expert in their offerings.

An error occurred while sending mail: The mail server sent an incorrect greeting: resomta-ch2-09v.sys.comcast.net resomta-ch2-09v.sys.comcast.net CSI IP 138.nnn.nnn.101 is not permitted to send messages. Please contact support if you feel this is in error.. it says my IP address blocked at spamhaus.org but I also have TB on another computer and it can send fine. (I put nnn in place of real IP numbers)