In A/B testing mode, a given recipient will only ever receive a single
email from a given mass-mailing campaign, no matter how many mailings
were sent to them.
This is useful for A/B-testing various mailings in order to check their
results. But is very difficult to understand for users who simply want
to organize their mailings into campaigns.
Additionally, the A/B testing option is hidden as a technical feature, so
it is hard to discover in order to troubleshoot unexpected number of
emails sent by a given mailing.
It will be safer to keep it off by default.
(keeping the parameter explicit, to better show that it is on purpose)
This partially reverts dee5c31264
Olivier Dony
fe2df80d73