The message is due to your IP or domain name being on a blacklist or appearing otherwise suspicious as spam. It could be a variety of issues, but it is not caused by your email client. One possibility is incorrect SPF and DKIM settings by your email provider. Did you post the complete error message? I ask that because I usually see additional text following the AUP# Did you make any software changes, such as addition of an antivirus product that forces an ad into the sig?

Thank you for this fast response. Yes I believe I did paste the entire error message, but here is another below. What I don't understand about it being an IP issue is that all of my email accounts are connected to the same IP, but it is only the one account causing issues. We didn't make any software changes but we did send out a large, blast email campaign Tuesday, (as we do every month), and there were probably some SPAM reports as a result. The reason I am looking for help with Thunderbird is that I sent an email to one of my employees as a test through webmail and it went right through. Then sent the exact same email from Thunderbird and it bounced. However, this email bounce to multiple employees below was sent through webmail. So I am not seeing a pattern other than nothing sent from Thunderbird (that one, specific account) works.

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

 sam.mclellan31759@bigcontactsemail.com
   (ultimately generated from sam.mclellan@coachdeck.com)
   host eig-east.smtp.a.cloudfilter.net [3.228.35.199]
   SMTP error from remote mail server after end of data:
   550 s1dvpnaiiYXE1s1dwpv3xA - <brian@coachdeck.com> message rejected AUP#SNDR
 tom.waters7298@bigcontactsemail.com
   (ultimately generated from tom.waters@coachdeck.com)
   host eig-east.smtp.a.cloudfilter.net [3.228.35.199]
   SMTP error from remote mail server after end of data:
   550 s1dvpnaikYXE1s1dwpv3xB - <brian@coachdeck.com> message rejected AUP#SNDR
 tew95@aol.com
   (ultimately generated from tom.waters@coachdeck.com)
   host eig-east.smtp.a.cloudfilter.net [18.215.58.191]
   SMTP error from remote mail server after end of data:
   550 s1dvp0Fv7eGxUs1dwpzkBo - <brian@coachdeck.com> message rejected AUP#SNDR
 kurt7288@bigcontactsemail.com
   (ultimately generated from kurt@coachdeck.com)
   host eig-east.smtp.a.cloudfilter.net [3.228.35.199]
   SMTP error from remote mail server after end of data:
   550 s1dvpKDv9c27Ds1dwpl9sP - <brian@coachdeck.com> message rejected AUP#SNDR
 stella.adenodi32830@bigcontactsemail.com
   (ultimately generated from stella.adenodi@coachdeck.com)
   host eig-east.smtp.a.cloudfilter.net [3.228.35.199]
   SMTP error from remote mail server after end of data:
   550 s1dvp7sctln5Qs1dwpjPZf - <brian@coachdeck.com> message rejected AUP#SNDR
 brian7287@bigcontactsemail.com
   (ultimately generated from kurt@coachdeck.com)
   host eig-west.smtp.a.cloudfilter.net [34.223.136.48]
   SMTP error from remote mail server after end of data:
   550 s1dvpP8pNGR2hs1dwp4599 - <brian@coachdeck.com> sender rejected AUP#CDRBL
 ed.duax7300@bigcontactsemail.com
   (ultimately generated from ed.duax@coachdeck.com)
   host eig-west.smtp.a.cloudfilter.net [34.217.196.71]
   SMTP error from remote mail server after end of data:
   550 s1dvpmnZbx1YBs1dwpFhnJ - <brian@coachdeck.com> sender rejected AUP#CDRBL

Reporting-MTA: dns; gator3197.hostgator.com

Action: failed Final-Recipient: rfc822;ed.duax@coachdeck.com Status: 5.0.0 Remote-MTA: dns; eig-west.smtp.a.cloudfilter.net Diagnostic-Code: smtp; 550 s1dvpmnZbx1YBs1dwpFhnJ - <brian@coachdeck.com> sender rejected AUP#CDRBL

Action: failed Final-Recipient: rfc822;kurt@coachdeck.com Status: 5.0.0 Remote-MTA: dns; eig-west.smtp.a.cloudfilter.net Diagnostic-Code: smtp; 550 s1dvpP8pNGR2hs1dwp4599 - <brian@coachdeck.com> sender rejected AUP#CDRBL

Action: failed Final-Recipient: rfc822;stella.adenodi@coachdeck.com Status: 5.0.0 Remote-MTA: dns; eig-east.smtp.a.cloudfilter.net Diagnostic-Code: smtp; 550 s1dvp7sctln5Qs1dwpjPZf - <brian@coachdeck.com> message rejected AUP#SNDR

Action: failed Final-Recipient: rfc822;kurt@coachdeck.com Status: 5.0.0 Remote-MTA: dns; eig-east.smtp.a.cloudfilter.net Diagnostic-Code: smtp; 550 s1dvpKDv9c27Ds1dwpl9sP - <brian@coachdeck.com> message rejected AUP#SNDR

Action: failed Final-Recipient: rfc822;tom.waters@coachdeck.com Status: 5.0.0 Remote-MTA: dns; eig-east.smtp.a.cloudfilter.net Diagnostic-Code: smtp; 550 s1dvp0Fv7eGxUs1dwpzkBo - <brian@coachdeck.com> message rejected AUP#SNDR

Action: failed Final-Recipient: rfc822;tom.waters@coachdeck.com Status: 5.0.0 Remote-MTA: dns; eig-east.smtp.a.cloudfilter.net Diagnostic-Code: smtp; 550 s1dvpnaikYXE1s1dwpv3xB - <brian@coachdeck.com> message rejected AUP#SNDR

Action: failed Final-Recipient: rfc822;sam.mclellan@coachdeck.com Status: 5.0.0 Remote-MTA: dns; eig-east.smtp.a.cloudfilter.net Diagnostic-Code: smtp; 550 s1dvpnaiiYXE1s1dwpv3xA - <brian@coachdeck.com> message rejected AUP#SNDR

The fact that you received those errors from webmail indicates that your email host may have inadvertently changed settings for your account. I am no expert on 550 errors, but that's a possibility. I encourage presenting those webmail errors for their assessment. The main difference between your PC and the email host is that you have a different IP address.

Thanks. I have sent them the errors and have chatted/spoken with them for hours. They never accept blame, they always tell me it is my ISP or, in this case, email client. Again, I really appreciate the assistance

Well, it could be the ISP, since your IP is one of theirs. Definitely worth pursuing. And, I regret that I am not more fluent in this. There are a few others on this forum and my hope is that they see this thread and offer other input.