That is a bigger problem than you think because text emails have a big problem also.
The spam filters read the text and exclude mails that have certainn trigger words – which words varies from mail filter to mail filter.
Casino, special, dear friend, $$, !!, and a ton of other things will trigger the filter and put your message in the spam box.
If you can’t send images anymore, you really can’t send anything at all anymore.
I see some very very short casino text messages are getting through. Must be someone sorted the addresses by ISP and adjusted each group to the filters.
What a pain, this also applies to the mailing list I compiled over the last 3 years, that is completely double opt in, answer the letter and get back to me, and easy opt out. This is a subscription newsletter and as clean as a list can get. It is incresingly bouncing – people can’t even get what they subscribe to anymore.
On the other hand, my mailbox is full of porn and mortgage and other crap mails.
This situation has gotten totally out of hand. Only real spammers employing methods unknown to me are getting through anymore.