Description of problem:
I upgraded from Fedora 24 to Fedora 25 over the weekend. Previously, sending an email from Evolution where the signature (developed in April 2013) contained lines like:
<IMG SRC="file:///home/louis/.local/share/evolution/signatures/red-dot.gif" WIDTH="7" HEIGHT="11" ALIGN="bottom" BORDER="0">
<B>Louis van Dyk</B>
would show the line with the image and the text with my name following that.
Since the upgrade users of Outlook report getting an error when opening one of my emails:
If I send an email to my GMAIL account, the red dot is not there - where it used to show before.
So I recreated my signature with the current Evolution, then I noticed that now the new file has the image MIME-base64 embedded in the signature file. A brilliant approach!
However, for those of us who are not aware of it, the signature is broken and causes chaos amongst the Outlook crowd.
I don't know what to suggest here, other than maybe a pop-up warning telling the email author that his signature no longer conforms and needs to be converted, possibly even doing it for him?
Version-Release number of selected component (if applicable):
Every time when using the old email signature.
Steps to Reproduce:
1. Using an old signature referring to a physical image file on the filesystem, write a message including it.
2. Outlook users get an error looking for the file in the file path mentioned in the signature, and GMAIL users just don't see the image.
3. Recreate the signature file in Evolution, and the new file contains the embedded image and works.
Some warning to the unsuspecting Evolution message author, or guidance in converting his signatures would make it more "Linux Desktop User Friendly".
Thanks for a bug report. I agree, it's a shame. The right approach would be to attach such images transparently, thus no change is required from the user.
Created commit 2c037de in evo master (3.23.90+) 
Created commit d8090af in evo gnome-3-22 (3.22.5+)