I sent a 10.61mb file succesfully, received it via my gmx account. I then tried one approaching the limit given by gmail of 25mb, my file was 24.80mb, this was also successfully sent and received. Both files were mp3’s. The 10mb limit notification was displayed in both cases.
If anything, perhaps the notification within the mail could be updated to read 25mb as the limit.
I finally managed to solve the problem by updating the google account authentication in the system settings.
So, it was a side effect of a problem with google authentication preventing to send any email…