Long time ago i had the same , only other prestashop version
it was at client side server related , so not prestashop !
to solve temporary i used the php mail function so he could use the shop , the forms wil work !
and i had the time to find out where the problems came from ,
perhaps this is for short a tempory solution to use php mail function ,
and or it is like already said a php 7.2 issue ,
but i don't have any problems in my own shop ,, with php 7,2
you can try to switch to at php 7.0 or php 7.1
what are the settings u are using ? are they right ?
do u use imap or pop ?
for now you can use PHP Mailfunction so you can go on with your shop