pop is an incoming mail protocol, so my guess is you have your account settings incorrectly configured.

Could you please do the following:

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

Today, my ability to send e-mails is all of sudden working again - without me changing anything. Perplexed but pleased.

Matt said

pop is an incoming mail protocol, so my guess is you have your account settings incorrectly configured. Could you please do the following:

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

Application Basics: Name: Thunderbird Version: 68.4.2

   Build ID: 20200123140255; Update Channel: release
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.4.2; 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) imap.gmail.com:995, SSL, passwordCleartext
     OUTGOING: , gmail.com:587, alwaysSTARTTLS, none, true
   account2: INCOMING: account2, , (none) Local Folders, plain, passwordCleartext
   account3: INCOMING: account3, , (pop3) mail.salisburytransitioncity.org:110, plain, passwordCleartext
     OUTGOING: , smtp.salisburytransitioncity.org:587, plain, passwordCleartext, true
   account4: INCOMING: account4, , (imap) mail.salisburylibdems.org.uk:143, alwaysSTARTTLS, passwordCleartext
     OUTGOING: , mail.salisburylibdems.org.uk:587, alwaysSTARTTLS, passwordCleartext, true
   account5: INCOMING: account5, , (imap) imap.gmail.com:993, SSL, OAuth2
     OUTGOING: , gmail.com:587, alwaysSTARTTLS, none, true
 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
       Chambers (UK); 1.0; true; chambers-en-GB@search.mozilla.org
       DuckDuckGo;1.0; true; ddg@search.mozilla.org
       Google; 1.0; true; google@search.mozilla.org
       Lightning; 68.4.2; true
       {e2fda1a4-762b-4020-b5ad-a41df1933103}
       Provider for Google Calendar; 68.2.1; true
       {a62ef8ec-5fdc-40c2-873c-223b8a6925cc}
       Twitter; 1.0; true; twitter@search.mozilla.org
       Wikipedia (en);1.0; true
       wikipedia@search.mozilla.org; Security Software
   Type: Name; Antivirus: Windows Defender Antivirus
     Antispyware: Firewall: Windows Firewall
 Graphics; Features; Compositing: Basic; Asynchronous an/Zoom:none
     WebGL 1 Driver WSI Info: EGL_VENDOR: Google Inc. (adapter LUID: 0000000000008e71)

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     WebGL 1 Driver Renderer: Google Inc. -- ANGLE (Intel(R) HD Graphics Direct3D11 vs_5_0 ps_5_0)
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I hope I got it all because when I copied it, I was given the choice to paste it, which created a list that was too long to submit, so I have edited out some of the white spaces. Thank you, Mike. mike.lennard876@gmail.com

'Incorrect greeting' means the authentication is incorrect for the account. You currently have authentication = none for both gmail accounts' outgoing servers. Tools/Account Settings, Outgoing Server (SMTP) at the bottom of the left pane, Edit the smtp servers for the gmail accounts. Change the authentication for the POP account to 'normal password' and OAuth2 for the IMAP account.