Nextcloud Notification e-mail being rejected by Shaw Smtp server

[Smith_oo4]

I use my shaw.ca email account to send notifications from some self-hosted services I run for my family. To the most part this works well and has not been an issue. However, I have never been able to get it to work with my self-hosted Nextcloud. Below are the email server settings I am using:

The following is the Debug log when I send a test e-mail in Nextcloud:

[core] Debug: < 220 shw-obgw-4002a.ext.cloudfilter.net cmsmtp ESMTP server ready

> EHLO nextcloud.example.com

< 250-shw-obgw-4002a.ext.cloudfilter.net hello [68.146.132.242], pleased to meet you

< 250-AUTH LOGIN PLAIN

< 250-SIZE 30000000

< 250-8BITMIME

< 250-STARTTLS

< 250 OK

> STARTTLS

< 220 Ready to start TLS

> EHLO nextcloud.example.com

< 250-shw-obgw-4002a.ext.cloudfilter.net hello [68.146.132.242], pleased to meet you

< 250-AUTH LOGIN PLAIN

< 250-SIZE 30000000

< 250-8BITMIME

< 250 OK

> AUTH LOGIN

< 334 VXNlcm5hbWU6

> c21pdGhfb280

< 334 UGFzc3dvcmQ6

> Q2FIQHZNc0R2OFIyI2Z3

< 235 ... authentication succeeded

> MAIL FROM:<username@shaw.ca>

< 250 <username@shaw.ca> sender ok

> RCPT TO:<username@gmail.com>

< 250 <username@gmail.com> recipient ok

> DATA

< 354 OK

> .

< 550 <username@shaw.ca> message rejected. AUP#SNDR

Any idea on why the message is being rejected or how I could try and better debug this?