This is the olny staically called method from the service. In this commit I refactored it to non-static so that it works properly with injected services.
[MAILPOET-5191]
Deliverability is not important in any of these tests, so it should be
safer and more reliable to use blackhole@mailpoet.com instead of the old
mailinator address, which was used before the blackhole@mailpoet.com
address existed.
[MAILPOET-4064]
