I have been using Microsoft Outlook and Shaw for 20 years on a PC without any issues.

My email seems to work fine except for when I send to a larger group of people (this is a new issue!).  Quick background...every few days, I'll send an email to a group of people consisting of 15-20 email addresses (never had an issue...been doing this for years).  This isn't spam or anything (just sending an email to players and coaches on a team that I head coach).  Now as of this weekend, I'm getting an error message that says:

From:  System Administrator

Subject:  Undeliverable (...it shows my original subject line...)

Your message did not reach some or all of the intended recipients.

The following recipient(s) cannot be reached:

      'xxxx.xxxxx@icloud.com' on 2/8/2022 9:18 AM

            Server error: '450 HTAtnu1WAa4s1HTAtnEk81 internal error. AUP#SMTP'

The email goes on to list 10 email addresses.  I'm assuming the email went through to the other emails that it didn't list.  The list of addresses that it says it didn't reach included icloud.com, gmail.com, outlook.com, and a couple businesses.  Not sure if it matters but I see it listed the same 10 email addresses in the last two times I received the error.

I called Shaw Technical Support.  They said they can't provide any assistance on a third party app such as Outlook (but I don't think this is an Outlook issue as the email left Outlook and then had issues on Shaw's server?).  They did confirm I wasn't anywhere close to my quota limit.  They suggested I try using Shaw Webmail which I did and it seemed like the group email worked.  However I really want to get Outlook working for me again.  Anybody have any ideas?

In case it helps, here is some info on my system...

I'm using Microsoft Outlook 2010 on a Windows PC running Windows 10 version 21H2. (Windows Update says I'm up to date)

I haven't done any recent updates (to my knowledge) or changed settings.

When I go into Outlook settings, it says the following for Server Information:

Account Type: POP3

Incoming mail service: pop.shaw.ca

Outgoing mail server (SMTP): mail.shaw.ca

When I go into Advanced settings, I see the following server port numbers:

Incoming server (POP3): 110

Outgoing server (SMTP): 25