Hi EwaG,
I am now able to make a reply post, so I wanted to include the response here as well.
You are correct, you are not blacklisted. The issue is that the server you are using is not authorized to send mail for the domain you are using.
Unfortunately it is difficult for me to go into too much detail as I don't have any information on the email addresses involved. But I can explain in generalities. I will use sender.com and recipient.com as example domains.
Sender Policy Framework, or SPF is a way of proving an email is authentic. The domain you are using to send mail from - sender.com has an SPF Policy in place that says that if we are checking SPF Policies, then we should reject any email that doesn't come from specific servers identified in the policy.
The domain you are sending to - recipient.com (our customer) has configured their services to check SPF Policies of incoming mail.
When the mail comes in, we check the SPF Policy as they want us to do and we see that the sending server isn't authorized for sender.com. We also see that sender.com wants us to discard such messages, and so we do.
There are a few ways you can resolve this issue:
1. Instead of using the cogeco.ca smtp server, use the server that is authorized in the SPF Policy of the domain you are using to send the mail.
2. Have the cogego.ca smtp server added as authorized in the SPF Policy of the domain you are using to send the mail
3. Send the mail using a domain that does not have an SPF Policy or that has an SPF Policy authorizing the cogeo.ca smtp server
Regards,
Todd Weltz
Senior Technical Support Engineer
Gateway Security Group
www.symantec.com