After several months of expierence, I could not figure out clearly what email address have been abusively tagged as hardbounce whereas they are still valid address.
I first thought that analysing the types of SMTP reply code could lead to identify the grey zone, but I must admit that this lead me nowhere.
The solution I finally chose is to work with a technological partner specialized in testing email address. I know that there are some service providers in US that offer similar service , but I did not tried them. The one I work with is in Europe and provide truly accurate information about email address validity. There are three steps :-1- Of course there is a first syntax analysis, -2- then a verification that the domain exists and is configured for email boxes, -3- then the final verification is to validate whether the contact exists with the corresponding email address. Valid addres tested is 100% valid and unvalid address is 100% really invalid, there is still a "grey zone" where the server is configured not to reply.
This check outside Eloqua system is interesting because it is a cross-check based with a reliable technology. This can help to reset valid email address abusively set as hardbounce, and you can rely on Eloqua information for email address where the testing cannot provide any answer. The testing and checking of email address is really accurate and sophisticated : it is able to test servers individually to prevent mass mailing filters to reply misleading informations.