Lots of emails won't go through. Says they are spam when they are not.

[Quack]

For the last two months, a lot of my emails to a person I've emailed a lot, and who does not have Shaw, suddenly stopped going through. They have supposedly been returned because of spam but these emails do NOT contain spam in any way. A search of IP addresses in the return message comes up clear when checking blacklisted IP addresses, except for one that indicates Shaw which shows blacklisting on some sites. The message I get back when the email doesn't go through, indicates "Client host [Shaw IP number]...blocked." That IP is the same Shaw IP.
The message I get back also says: Threat Look Up URL referring to the an IP checking website which this message won't let me include in the message. When I look up that Shaw IP in that website, it shows a threat level due to "Spam sources".
Does this mean that this Shaw IP is known for marking spam and therefore is blacklisted? Why would my emails be blocked in this way when they are clearly not spam. Some times I can have 4-6 emails return...and then suddenly one goes, then more don't go. There's no reason why these emails should be blocked.

[rstra]

You would need to contact the recipient’s email provider. 

[Quack]

If that's the case, why is it showing that Shaw is the IP that is causing spam issues.   It doesn't look like it's making it to the other provider. 

This is the message I am getting:

Service unavailable; Client host [Shaw IP entered here] blocked using rbl.br1.megarouting.com; Threat Found: Spam Sources / Threat Look Up URL: https://www.brightcloud.com/tools/url-ip-lookup.php
When I look up the Shaw IP  on a few websites, it comes up that that IP has been blacklisted, and on the brightcloud site it shows that IP is a threat regarding Spam. This is why it appears to be a Shaw issue.

I spoke to Shaw a couple of days ago. I thought the person may have said they reset my connection but I'm not sure. But suddenly all emails have gone through over 12 hours, then until tonight, when they stopped again.

[Quack]

Just another point.....I recently started to send a number of the blocked emails through another web-based email account/server because they don't go from Shaw if retried, and none of them have been blocked so far. This says to me that it's not the email that's a problem, and it's not the email that is spam which I already know of course. It's the fact that it's coming from Shaw that seems to be the reason all these emails keep getting blocked.  What does that say?

[rstra · Accepted solution]

The email recipient’s email provider is blocking the Shaw IP, it is up to them to remove Shaw from the blacklist. 

[mdk]

@Quack --  It's the fact that it's coming from Shaw that seems to be the reason all these emails keep getting blocked.  What does that say?

It says that the receiving mail-server is checking a "black-list" of IP-addresses, and the IP-address present in your E-mail is listed on that "black-list".

Somebody (you, your recipient, Shaw, or the receiving Internet Service Provider) needs to work with managers of that "black-list" to de-list Shaw's IP-address, or the recipient Internet Service Provider needs to stop checking that "black-list", under the assumption that none of Shaw's customers ever send "spam" E-mail through that Shaw IP-address.