For start check exim queue:

'VPS hosting' page -> select string with your server -> press on 'Instructions' button on top. On new opened page 'Virtual server activation' find chapter 'Hestia LAMP control panel'.
Login in Hestia control panel -> press on gear icon on top -> press on 'Task Monitor' button -> scroll to 'Exim Usage' graph -> see on 'Curent' value on graph. This is count of messages in Exim MTA queue now.
If this value not 0 - some of your messages defferred. Pay attention, this graph updated once per 5 minutes.
For immideately show queue status press on 'View Advanced Details' button -> press on 'Mail' tab on top -> find 'Exim queue status' chapter (on top).

You can see queued messages in ssh too:

'VPS hosting' page -> select string with your server -> press on 'Instructions' button on top. On new opened page 'Virtual server activation' find chapter 'Server information'.

Connect via ssh with root password and run:

# mailq

For delete all messages from queue run:

# exim -bp | exiqgrep -i | xargs exim -Mrm