Mail Alert Virtual Product (download link) only works occasionally

[Mctijn]

A few days ago I finished my shop. People can purchase and download sewing patterns (zip files).

 

I tested it on several mail accounts and all works perfectly. I received the mail alert with the download link.

 

Now, many people (I guess 60-70% of the total orders) are complaining that they get the first mail alert (registration completed), but not the second (download link to the pattern).

 

Often I asked them to look in their spam folder, but no mail.

 

Is this is common problem? Does anybody have an idea?

 

Thanks a lot!

Edited February 26, 2015 by Mctijn (see edit history)