@chicuelo Many web hosts are insisting that form submission are only sent via SMTP - not the PHP sendmail function. Some even go to the trouble of disabling, or turning off PHP sendmail. Whilst it is usually possible to turn it back on again through cPanel, your host will most likely give you grief if you do.
Something that Blocs could do, to make life easier for those being troubled by this trend, is to offer another form sending option that uses SMTP. Below is a screenshot from another website app that does precisely this.
This option allows forms to be sent as normal emails via an email account set up on the server. Note to @Norm If an option similar to this could be incorporated into Blocs, our problems would be over.