Often times, gaming motherboard's tend to come with an app that helps managing which app on your platform gets higher priority. The issue you've mentioned seems to indicate that your ISP might be limited in it's capability for both download and upload traffic, whereby you're on a limited bandwidth package/plan. Might it be possible to mention the specs to your build and the max bandwidth for your ISP plan(both upload and download)?
The computer on which the mail is being sent is my father's laptop, so I don't know the specs, but it's definitely not a gaming computer, so it wouldn't have a gaming motherboard.
As for my connection, yeah, the problem is in part there definitely, because I have an asymmetric connection with 10/0.45 Mbps, but there are other programs I've used which use upload and by limiting the upload to 0.30 Mbps, I was able to have no problems while uploading full blast.
So, in spite of the low up bandwidth, the problem here is that the email client uses ALL of it, instead of just, say 80% to leave some for other purposes.
So, I'd need some way of telling it to not use all of the up bandwidth.