Kind of OT - Error when forwarding spam to Centurylink

[Chris]

This is probably pretty OT but I have to ask anyway just because the
idiots at my ISP think the problem is because I'm running Linux and
Evolution. Every time I try to forward a spam to their 
spam at centurylink.net address I get the attached screenshot. I ran this
Evolution debug command - CAMEL_DEBUG=smtp evolution >& embarq.txt and
the last line of the output reads to me that the issue is that their
server thinks the spam I'm trying to report is spam, of course it is.

[SMTP] Connecting to server smtp.embarqmail.com:25 from account 
1414156398.2275.3 at localhost
[SMTP] received: 220 smtp.centurylink.net ESMTP
[SMTP] sending: EHLO localhost
[SMTP] received: 250-smtp04.onyx.dfw.sync.lan says EHLO to
71.221.16.42:57414
[SMTP] received: 250-XDUMPCONTEXT
[SMTP] received: 250-AUTH=LOGIN
[SMTP] received: 250-AUTH LOGIN
[SMTP] received: 250-PIPELINING
[SMTP] received: 250-8BITMIME
[SMTP] received: 250-ENHANCEDSTATUSCODES
[SMTP] received: 250 STARTTLS
[SMTP] sending: STARTTLS
[SMTP] received: 220 2.0.0 continue
[SMTP] sending: EHLO localhost
[SMTP] received: 250-smtp04.onyx.dfw.sync.lan says EHLO to
71.221.16.42:57414
[SMTP] received: 250-PIPELINING
[SMTP] received: 250-XDUMPCONTEXT
[SMTP] received: 250-8BITMIME
[SMTP] received: 250-AUTH=LOGIN
[SMTP] received: 250-AUTH LOGIN
[SMTP] received: 250 ENHANCEDSTATUSCODES
[SMTP] sending: AUTH LOGIN
[SMTP] received: 334 VXNlcm5hbWU6
[SMTP] sending: Y3BvbGxvY2tAZW1iYXJxbWFpbC5jb20=
[SMTP] received: 334 UGFzc3dvcmQ6
[SMTP] sending: Q0Fwb1QxOTQ4
[SMTP] received: 235 2.0.0 Authed. Go on.
[SMTP] Sending with server smtp.embarqmail.com:25 from account 
1414156398.2275.3 at localhost
[SMTP] sending: MAIL FROM:<cpollock at embarqmail.com> BODY=8BITMIME
[SMTP] received: 250 SPF validation soft failure
[SMTP] sending: RCPT TO:<spam at centurylink.net>
[SMTP] received: 250 2.1.5 Accepted for you
[SMTP] sending: DATA
[SMTP] received: 354 3.0.0 continue.  finished with "\r\n.\r\n"
[SMTP] sending: \r\n.\r\n
[SMTP] received: 554 5.7.1 [P4] Message blocked due to spam content in
the message.

Or am I wrong that the issue is on their end?
-- 
Chris
31.11972; -97.90167 (Elev. 1092 ft)
16:02:08 up 7 days, 7:19, 1 user, load average: 1.48, 1.42, 1.10
Description:	Ubuntu 18.04.3 LTS, kernel 5.0.0-25-generic
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