Bonecrusher said

After sending,

Sending what? Where?

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Does not matter to whom sent/replied. Problem just reoccurred when in safe mode. I note that you refer firefox; problem is in Thunderbird.

My goof; wrong forum. Sorry

moved to the Thunderbird support queue from Firefox support

Info as requested:

 Application Basics

   Name: Thunderbird
   Version: 52.9.1
   User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20180709124824
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) pop.csi.com:995, SSL, passwordCleartext
     OUTGOING: , smtp.csi.com:587, alwaysSTARTTLS, passwordCleartext, true
     OUTGOING: , smtp.csi.com:587, alwaysSTARTTLS, passwordCleartext, false

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

   account4:
     INCOMING: account4, , (imap) imap-mail.outlook.com:993, SSL, passwordCleartext
     OUTGOING: , smtp-mail.outlook.com:587, alwaysSTARTTLS, passwordCleartext, true

 Crash Reports

 Extensions
   Lightning, 5.4.9.1, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   Bitdefender Antispam Toolbar, 7.0, false, bdThunderbird@bitdefender.com

 Important Modified Preferences

   Name: Value

     accessibility.typeaheadfind.flashBar: 0
     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
     dom.apps.reset-permissions: true
     extensions.lastAppVersion: 52.9.1
     
     gfx.blacklist.direct2d: 3
     gfx.blacklist.direct2d.failureid: FEATURE_FAILURE_DL_BLACKLIST_g984
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 88e68b72-7d5d-4e0b-815d-4d1cea7e351
     media.gmp.storage.version.observed: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1535321279
     places.history.expiration.transient_current_max_pages: 122334
     plugin.importedState: true
     

 Graphics

     GPU #1
     Description: Intel(R) G33/G31 Express Chipset Family
     Vendor ID: 0x8086
     Device ID: 0x29c2
     RAM: Unknown
     Drivers: igdumd64 igdumdx32
     Driver Version: 8.15.10.1930
     Driver Date: 9-23-2009

     Features
     Direct2D: Blocked for your graphics driver version.
     DirectWrite: true (6.2.9200.22164)
     ClearType Parameters: Gamma: 2.2 Pixel Structure: RGB ClearType Level: 100 Enhanced Contrast: 50
     WebGL Renderer: Google Inc. -- ANGLE (Software Adapter Direct3D11 vs_5_0 ps_5_0) -- OpenGL ES 2.0 (ANGLE 2.1.0.2a250c8a0e15)

     AzureCanvasBackend: skia
     AzureCanvasAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: skia

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.13.1
     4.13.1

     NSS
     3.28.6
     3.28.6

     NSS Util
     3.28.6
     3.28.6

     NSS SSL
     3.28.6
     3.28.6

     NSS S/MIME
     3.28.6
     3.28.6

Do you have Bitdefender scanning outgoing mail? If so, disable that option and see if the problem remains.

Disabled outgoing mail scan. No change. I did restart computer. When I try to close the successful send window, goes to "not responding". I did try something else, sent using the account that is IMAP, worked OK, so it is only the POP3 that fails. Since message is usually sent, do not think problem is at CIS (AOL) Looking at data I sent above, Acct 1 has double entry. Is that OK?

restart Thunderbird in safe mode and retry your send. Does it make a difference? Does composing in plain text change anything? Hold the shift key when you click to "write", you will notice the fonts part of the compose widow is missing.

No change. Also changed SMTP from CIS to AOL, that was not it. Progress; Problem does not occur if W7 is in safe mode. What does that suggest?

> Problem does not occur if W7 is in safe mode

Antivirus or some other software that is loaded when windows starts.

Have you determined what software it is??

My first thought was Malwarebytes, which is off in safe mode, but turning it off manually did not change problem. I have turned off the Bitdefender check on outbound, will try turning off more, but I think it loads in safe.

> think it loads in safe.

Bitdefender will not load if windows is properly started in safe mode

You are correct, will not load in safe mode. I will contact their support. Still does not explain why problem is only POP3 account, not IMAP! Am thinking it is AOL, and they seem to have dropped forums. Am about to just live with it.

Trouble ticket opened with them [Bitdefender].

Also, a new angle has appeared; when I try to delete a sent message, goes to "not responding" even if message was sent long before this problem. Is it possible that the "sent" folder is corrupted?

Also, if I "compact folder" it hangs the same. Repair folder did nothing.

I went to safe mode, deleted several messages that were complex, did compact, which now worked, and problem appears solved. The complex messages had a strange encryption signature in them.

Bonecrusher,

How did it go with Bitdefender  ??

was fixed 9/17, deletion of messages which had encrypted attachments.

This is interesting. how were the attachments encrypted?

I don't have a clue. They were papers on a house closing from a title company. No problem reading and digital signing, but they acted up afterward.