It's very annoying to the point I might have to leave Tbird.

I can believe it's annoying. Changing to a different email client might work for you, however, it wouldn't fix the underlying problem.

The certificate is not trusted because the issuer certificate is unknown.

You've already been given the cause of the problem. This error message is often caused by anti-virus software intercepting your secure connection to the server. It can also be caused by malware trying to spy on you or having you connect to a malicious server. It can also be a mis-configuration of your provider's server.

Can you inspect the certificate (see attached screenshot for instructions).

What's needed is the issuer of the cert you get.

Thanks. Here is the issuer:

dbm001.dcs.int.inet Zimbra Collaboration Suite Zimbra Collaboration Suite

I don't know what to do now. I'm running a vpn, perhaps that's the issue.

Also, I have Kaspersky anti virus on incoming messages only, and I just noticed that under "Connectivity" it is scanning imap/pops/ etc. traffic.

Here is the issuer: dbm001.dcs.int.inet

Can you post a screenshot? http://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem

From your OP you're trying to connect to imap.shaw.ca:143. Typically port 143 is not being used for secure connections.

From the server you do get a certificate issued for px005.dcs.int.inet, *.lab.dcs.int.inet, and issued by dbm001.dcs.int.inet.

Are you certain you do connect to the correct server?

Have you checked with your email provider about the correct server settings?

here's the screenshot

I'm not certain and I haven't contacted them yet for server settings.

Please confirm the correct server settings with your email provider and post the result.

Also, can you post your Troubleshooting Information? At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information. Press the Copy text to clipboard button and paste the information into your reply.

Voila. Thanks for all this.

 Application Basics

   Name: Thunderbird
   Version: 38.5.0
   User Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20151221142744
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (imap) imap.gmail.com:993, SSL, OAuth2
     OUTGOING: smtp.gmail.com:465, SSL, OAuth2, true

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

   account3:
     INCOMING: account3, , (imap) imap.shaw.ca:143, alwaysSTARTTLS, passwordCleartext
     OUTGOING: smtp.shaw.ca:587, plain, passwordCleartext, true

 Crash Reports

 Extensions
   Lightning, 4.0.5, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     browser.cache.disk.capacity: 358400
     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: 38.5.0
     font.internaluseonly.changed: true
     font.name.monospace.el: Consolas
     font.name.monospace.x-cyrillic: Consolas
     font.name.monospace.x-unicode: Consolas
     font.name.monospace.x-western: Consolas
     font.name.sans-serif.el: Calibri
     font.name.sans-serif.x-cyrillic: Calibri
     font.name.sans-serif.x-unicode: Calibri
     font.name.sans-serif.x-western: Calibri
     font.name.serif.el: Cambria
     font.name.serif.x-cyrillic: Cambria
     font.name.serif.x-unicode: Cambria
     font.name.serif.x-western: Cambria
     font.size.fixed.el: 14
     font.size.fixed.x-cyrillic: 14
     font.size.fixed.x-unicode: 14
     font.size.fixed.x-western: 14
     font.size.variable.el: 17
     font.size.variable.x-cyrillic: 17
     font.size.variable.x-unicode: 17
     font.size.variable.x-western: 17
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 612ec198-f1eb-4d94-b229-30ea52d7163
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1451291923
     places.history.expiration.transient_current_max_pages: 104858
     plugin.importedState: true
     privacy.donottrackheader.enabled: true

 Graphics

     Adapter Description: AMD Radeon(TM) R6 Graphics
     Vendor ID: 0x1002
     Device ID: 0x9874
     Adapter RAM: 512
     Adapter Drivers: aticfx64 aticfx64 aticfx64 aticfx32 aticfx32 aticfx32 atiumd64 atidxx64 atidxx64 atiumdag atidxx32 atidxx32 atiumdva atiumd6a atitmm64
     Driver Version: 15.100.1020.1009
     Driver Date: 5-7-2015
     Direct2D Enabled: false
     DirectWrite Enabled: false (6.3.9600.18123)
     ClearType Parameters: ClearType parameters not found
     WebGL Renderer: false
     GPU Accelerated Windows: 0

     AzureCanvasBackend: skia
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: cairo

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.10.10
     4.10.10

     NSS
     3.19.2.1 Basic ECC
     3.19.2.1 Basic ECC

     NSS Util
     3.19.2.1
     3.19.2.1

     NSS SSL
     3.19.2.1 Basic ECC
     3.19.2.1 Basic ECC

     NSS S/MIME
     3.19.2.1 Basic ECC
     3.19.2.1 Basic ECC

Change the IMAP server setting as per shaw.ca instructions. https://community.shaw.ca/docs/DOC-1652

If the problem is still seen after that, post your Troubleshooting Information once again. Just the 'Mail and News Accounts' section will do.

Ok. I've just chatted with shaw and we changed the server settings, so far so good. Thanks again!

Good. When your problem is fixed, can you mark the thread as 'Solved' please? Thank you.