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