I do not seem to receive all emails.

The show as sent from the sender, don't show in m in-box or my junk.  This happens a lot with on-line enquiries, orders and as I don't know what email address they are sent from I cant add to safe senders.  What is happening as it means I am missing important emails.  And it seems so random in its choice.  Where are they ending up if they have been shown as sent from sender and not arriving in in-box or junk??  Please help as this is a real problem for me.