Log onto your email account and go to the settings which are found by clicking on your username at the right hand side.
Once there go to "Mail" > "Blocked Senders" and scroll down checking to see if your contacts email addresses are there and also if the gmail domain is there.
If you find them, click on the box to the left and then "delete". This will remove them from your blocked senders list not your contacts list.
The other thing to do is add their email address to your "Safe Senders" list which is found next to "Blocked Senders".
Found gmail.com in “blocked senders” although don’t understand why.
Many thanks for the community help.