lets start at the begining.

1. Charter support is about as bad as it gets while still answering the phone. "script monkeys" is the derogatory term used to describe that kind of support.

2. Sending mail from the charter web interface is about as useful in diagnostic term as you taking mail to a post office box in the next town and dropping it in a post box there. It proves nothing about the actions of the local postal office (SMTP Server in email terms). It old demonstrates you found a work around for the issue.

3. My guess is your settings for the charter server beed to be updated for changes made by charter. But as I say, that is really only a guess.

Please add the troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

Please also add the result of these troubleshooting steps to your post

• Does Thunderbird work in TB Safe mode (see Thunderbird Safe Mode)?
• Do you use anti-virus and firewall software? What is the version?

As requested:

 Application Basics

   Name: Thunderbird
   Version: 52.7.0
   User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:52.0) Gecko/20100101 Thunderbird/52.7.0
   Profile Folder: Show in Finder
   Application Build ID: 20180322145026
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account1:
     INCOMING: account1, , (none) Local Folders, plain, passwordCleartext

   account2:
     INCOMING: account2, , (imap) mobile.charter.net:993, SSL, passwordCleartext
     OUTGOING: , smtp.charter.net:25, plain, passwordCleartext, true

 Crash Reports
   https://crash-stats.mozilla.com/report/index/bp-ca927365-eed2-4fe5-bc31-cca9b2130914 (9/14/13)

 Extensions
   Lightning, 5.4.7, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 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: 52.7.0
     font.internaluseonly.changed: false
     gfx.blacklist.dxva: 3
     mail.openMessageBehavior.version: 1
     mail.spotlight.firstRunDone: true
     mailnews.database.global.datastore.id: 2cb0fe2a-243c-9d42-8f96-96d03f6e2dd
     media.gmp.storage.version.observed: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     permissions.default.image: 2
     places.database.lastMaintenance: 1526942012
     places.history.expiration.transient_current_max_pages: 122334
     plugin.importedState: true
     plugin.state.silverlight: 0
     print.macosx.pagesetup-2: PD94bWwgdmVyc2lvbj0iMS4wIiBlbmNvZGluZz0iVVRGLTgiPz4KPCFET0NUWVBFIHBsaXN0IFBVQkxJQyAiLS8vQXBwbGUvL0RURCBQTElTVCAxLjAvL0VO…
     print.print_bgcolor: false
     print.print_bgimages: false
     print.print_colorspace:
     print.print_command:
     print.print_downloadfonts: false
     print.print_duplex: -437918235
     print.print_evenpages: true
     print.print_in_color: true
     print.print_margin_bottom: 0.5
     print.print_margin_left: 0.5
     print.print_margin_right: 0.5
     print.print_margin_top: 0.5
     print.print_oddpages: true
     print.print_orientation: 0
     print.print_page_delay: 50
     print.print_paper_data: 0
     print.print_paper_height: 11.00
     print.print_paper_name:
     print.print_paper_size_type: 1
     print.print_paper_size_unit: 0
     print.print_paper_width: 8.50
     print.print_plex_name:
     print.print_resolution: -437918235
     print.print_resolution_name:
     print.print_reversed: false
     print.print_scaling: 1.00
     print.print_shrink_to_fit: true
     print.print_to_file: false
     print.print_unwriteable_margin_bottom: 56
     print.print_unwriteable_margin_left: 25
     print.print_unwriteable_margin_right: 25
     print.print_unwriteable_margin_top: 25
     print.printer_Fax.print_bgcolor: false
     print.printer_Fax.print_bgimages: false
     print.printer_Fax.print_evenpages: true
     print.printer_Fax.print_footercenter:
     print.printer_Fax.print_footerleft: &PT
     print.printer_Fax.print_footerright: &D
     print.printer_Fax.print_headercenter:
     print.printer_Fax.print_headerleft: &T
     print.printer_Fax.print_headerright: &U
     print.printer_Fax.print_in_color: true
     print.printer_Fax.print_margin_bottom: 0.5
     print.printer_Fax.print_margin_left: 0.5
     print.printer_Fax.print_margin_right: 0.5
     print.printer_Fax.print_margin_top: 0.5
     print.printer_Fax.print_oddpages: true
     print.printer_Fax.print_orientation: 0
     print.printer_Fax.print_reversed: false
     security.sandbox.content.tempDirSuffix: {2a678103-6405-6047-941b-253dd19d093a}

 Graphics

     GPU #1
     Vendor ID: 0x8086
     Device ID: 0x0166

     Features
     WebGL Renderer: Intel Inc. -- Intel HD Graphics 4000 OpenGL Engine -- 2.1 INTEL-10.32.48

     AzureCanvasBackend: skia
     AzureCanvasAccelerated: 0
     AzureFallbackCanvasBackend: none
     AzureContentBackend: skia
     TileHeight: 512
     TileWidth: 512

 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

Right click your account in the foldr pane. Select settings. At the bottom of the list of accounts is "Outgoing server (SMTP)". Select it.

in the list of outgoing servers on the right (it might bea list of one), select your charter outgoing server and click the edit button.

Change the port to 587 Change connection security to SSL/TLS

If that does not work, repeat and change the server name from smtp.charter.net to mobile.charter.net