Yes,
On the postmaster's website, they say the sender IP is unlocked after a few hours and then, it can retry to send more messages. But it keep blocking after a few messages.
Also have a "4.5.0", "too many errors" that is not listed in any help or information on the website.
I'll keep changing the max smtp out and max msg per connection. =/
Any news, I'll tell you!
4.5.0 Too many errors means you have to many hardbounces in your list, probably also causing a low reputation and hence why you
get blocked so quickly.