Do you have access to a webmail interface? Can you successfully send from there?

I have tried webmail with the same result (recipients to not receive the email).

It is interesting that I can send emails using my gmail account from this computer and can send from my cell phone but not from my desk top or laptop.

I have tried webmail with the same result (recipients to not receive the email).

That suggests that there's a problem with your mail service, rather than with Thunderbird itself. Or... I don't know how many recipients you're talking about, or of they are all using different mail services, but if just one maybe THEY are having a problem on their end or have blocked your host.

No recipients can receive my sent emails (i.e. all recipients). I agree that the problem certainly appears to be with the email service (Mozilla Thunderbird) but I don't know where to look to determine where the problem is (or how to resolve it).

Are you referring to "Thundermail", Mozilla's upcoming(?) mail service? I have heard about it, but I didn't think it was available to the public yet. Are you part of their beta team? "Thunderbird" is strictly a mail agent/client that interfaces with your Internet email provider (Gmail, Xfinity, AOL, etc.)

Over a year ago we changed to IMAP/Internic for our emails but I still used the Thunderbird for my desk top and laptop. Others in the company use Outlook with the IMAP setup for the emails and they are not having problems sending/receiving emails. From the "About Thunderbird", I use the Thunderbird version 140.2.0esr.

OK, that makes more sense. But you said you can't successfully send email from your webmail interface. At that point, Thunderbird is out of the picture, which suggests to me that the problem is with Internic or downstream from there.

OK. I did have a "chat" with Internic yesterday and I'm on an "expedited" service with them to try and resolve the problem. ETA for their next reply was supposed to be 24-72 hours. No reply back from them yet.

So you are using mail hosted by internic. According to the DNS records for your domain https://intodns.com/hycotec.on.ca you are also using the services of https://www.vadesecure.com/en/ but I see nothing in the DNS for smtp.internic.ca being a permitted sender for the domain in the MX records, despite internic telling you to use it. So my guess is this is entirely a problem created by your hosting provider and someone who has signed up for enhanced email security.

Why internic would be advertising an email address at official website of NBC as the technical contact for the domain I have no idea. https://www.internic.ca/whois/hycotec.on.ca

So I am seeing some sort of corporate mess that I am not qualified to untangle. I am thinking that there may be more ideology in this setup than there are facts.

However despite their assertion that these are correct settings. Incoming:

   IMAP
   Server hostname: imap.internic.ca
   Port: IMAP (993)
   SSL: SSL/TLS
   Authentication: Normal Password

Outgoing:

   Server hostname: smtp.internic.ca
   Port: SMTP (587)
   SSL: STARTTLS
   Authentication: Normal Password

I have doubts about the server (smtp.internic.ca) actually being correctly configured in DNS to send mail for hycotec.on.ca. It is not mentioned in the MX record for the domain greatly increasing the chance a recipient server will block it as spam.

There is none of the modern delivery validation mechanisms configured.SPF,DKIM or DMARC See https://mxtoolbox.com/emailhealth/hycotec.on.ca/

So delivery to most major mail providers will be more than a little problematical. You need someone to configure email on the domain if you actually want it to be delivered to customers inboxes instead of the spam folder, or not at all. This is not a Thunderbird issue I do not think as fundamental domain configuration information is either incorrect of missing entirely. This is a DNS issue in my opinion.