Forwarding emails with large attachment limits

[#1]

I received an interesting Fwded email recently covering the current construction methods of the Morgan Car Co., This had in its text 23 images and the size of the whole email was just less than the 7 MB limit for attachments. The email came from another Plus.net user who tells me that he has not had any difficulty with forwarding it to several others as well as myself. In my case, using webmail to do the same, it just will not send it and did not specify why. I noticed that the images now appear as though they were attachments in the left-hand box. Any clues about this oddity out there?

[#2]

I often receive emails from another Plusnet user, which can total over 5MB, and I think have exceeded 7MB. Mostly these consist of a single attachment which is a PowerPoint presentation with a couple of dozen image slides. They are sent via relay.plus.net using MS Outlook 2003, copied to a small no of recipients, without problems,
From memory, during Roundcube beta trials I think it was determined that inline images received from elsewhere were listed as attachments, but if one forwarded the messages they still appeared inline. Standard attachments remained like that. I think this was attributed to images needing to be mime-encoded for transmission - the structure of the document sorted out the inline/attachment differences.

[#3]

Thanks for that. I do get quite a lot of Presentations which could easily pretty large but none, so far, have failed apparently due to excess sizes. I have looked at the email itself and it shows its size as 6.7MB. Clearly it would not take much to hit the buffers but it was clearly quite OK for the sender of it to me unscathed as I mentioned before.
I have found that the 7MB limit on attachments appears to be strictly applied. I had 4 images from my camera each of about 2.5MB but, on sending only the first two were included and I had to send a second email with the other two attached. I continue to be puzzled. I am using the the Roundcube webmail together with Firefox 18 and Win 7 if that makes any difference.

[#4]

I've just sent an email with an 18.7MB pdf attached to an external address using Thunderbird on Windows Vista. The sent folder says the email size is 25.6MB which seems about right for mime encoding. This arrived safely.
Using Roundcube, the "attachment" prompt warned of the 7MB limit and wouldn't upload that file. However I selected a 6.4MB wmv file and a 6.3MB pps; both these uploaded to the same email OK and were sent and received at the external address successfully. The message size in the sent folder is 17.1MB.
This is using FF 18 on Windows Vista. I'll try forwarding the message later this evening.
Edit.
Forwarded 17.1MB message (with the two (originally) 6.4MB and 6.3MB attachments) using webmail. No problem sending and received successfully. So not sure why you should be having problems.