Hallo MIchael! Thank you for your quick reply much appreciated.
In WHM Server > TWEAK SETTINGS so I guess this is globally. Where should I look for "account specific"?
Anyway in our WHM Server > TWEAK SETTINGS we have
Max hourly emails per domain [?]
Unlimited default
The percentage of email messages (above the account’s hourly maximum) to queue and retry for delivery. (Minimum: 100; Maximum: 10,000) [?]
125 % default
Count mailman deliveries towards a domain’s Max hourly emails. [?]
Off
default
Maximum percentage of failed or deferred messages a domain may send per hour [?]
Unlimited default
But reading some topics on this forum I found everyone saying that setting "Maximum percentage of failed or deferred messages.." to unlimited won't avoid the block to kick in and this is exactly what we experienced so far