Why does Yahoo"s SpamGuard consider these IPs to be spamming?

[Disclaimer: Somewhat 'off'-topic, but since I'm a 'regular' in this
group, ...]

For some of my extra mail accounts, I use Yahoo! Mail which I POP with
YPOPS!.

Lately I 'missed' some mail, i.e. I expected mail but didn't get it.
When looking into it, I found that Yahoo's SpamGuard was on by default
and had put some valid/non-spam messages in my Bulk folder (which I
don't POP).

I looked at the headers of the false positives and they seem to be
considered spam because of their 'originating' IP. One example was:

> X-Rocket-Spam: 38.113.6.61
> X-YahooFilteredBulk: 38.113.6.61
> X-Originating-IP: [38.113.6.61]

38.113.6.61 is sneakemail.com. The (solicited personal, i.e. not bulk)
mail had been sent to one of my Sneakemail addresses (which 'forwards'
to one of my Yahoo addresses).

Another one was:

> X-Rocket-Spam: 67.55.32.2
> X-YahooFilteredBulk: 67.55.32.2
> X-Originating-IP: [67.55.32.2]

67.55.32.2 is host144.host144-server.com which (apparently) is the
mailing service used by an alumni organization of which I am a member.
The mail was bulk, but not unsolicited, i.e. it wasn't spam.

Can you explain why these IPs would be considered likely spam-sources?

Thanks in advance for any and all responses.

[I checked a few blocking lists, but did not find one which listed these
IPs.]