Specific email will not send

I have had this happen occasionally.

One specific email will not send. Other emails do sent normally & I receive emails normally. It is not the recipient since I can send other emails to that same recipient.

Today, I tried to send an email to someone with 4 links.

Kept getting this error:

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Cannot send message using the server Cox

Sending the message content to the server failed.

The server response was: <my email address@cox.net> message rejected. Refer to Error Codes section at https://www.cox.com/residential/support/email-error-codes.html for more information. AUP#CXSNDR

Select a different outgoing mail server from the list below or click Try Later to leave the message in your Outbox until it can be sent.

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I tried sending another email with the same ‘body’ & 4 links to myself. It failed with the same error message.

I finally broke the 4 links into 4 separate emails to send to that recipient & all but 1 went through. So, one of the links was causing that error.

The URL ends with ASPX. Is there some issue where Mac Mail rejects links with that domain name extension?

This is the link that stopped the email from sending:

https://www.illinoisworknet.com/ApprenticeshipIL/Pages/default.aspx