[3.6.4]Can't send e-mail

[crashys]

After someone registers at my forum, in the registeration period, while they are waiting for the e-mail, they receive nothing...but I can send e-mails by the adminCP, I'v tried with multiple e-mail clients and its always the same thing, I haven't changed any of those configurations in the adminCP nor deleted any files related to that.
Any help would be appreciated, thanks in advance.