Attachments not arriving for Cox user

Question

My wife's sister uses Cox as her ISP. We use Comcast.
My wife has been trying to send her sister some pictures of the kids. Her sister receives the email with text, however, no attachments.
As a test, my wife sent the same email to her sister and copied me at work. I use MS Outlook on an Exchange server using Windows XP Pro. I receive the email & attachments just fine. So obviously the attachments are leaving the computer ok. Her sister still can not receive them.
As another test, I reply to the email my wife sent and include her sister in the message. She receives my reply as well as the attached images.
I am convinced that Cox is stripping the attachments from my wife's emails but I can't explain why her attachments don't go through yet mine from work do. It's the same email with the same attachments. Of course, my wife's emails are coming from Comcast and mine from my work's SMTP.
The email size is under the limits by Cox. My wife has always sent pictures to her in the past. It seems this problem started when we bought our iMac.
She even tried sending the pictures using "Windows Friendly Format" but it still didn't work.
Any ideas on what's happening and if there's actually anything we can do about it? Or is this a Comcast/Cox thing?

Answer 1

In the multiple emails my wife tried to send to her, she tried two different methods.
1. Within iPhoto, choosing the pictures she wants to send and then clicking the Email button to create a new Mail message with the pictures attached.
2. In Mail, creating a new message and then manually attaching the pictures through the Finder. iPhoto is not even involved.
Neither method worked for her sister. Both methods worked for me at work.
If you like, we can arrange for a series of tests so we can try and identify what the problems are and document them for everyone's benefit. Hopefully it will help insight some fixes from Apple.