Aimbox.com

Can you send emails from:
a – email account without any problems?
b – from a php script on your site.

Your IP address might be blacklisted or your contact form is not configure properly. Could you provide more detail?

1 – If you are going to use FTP just overwrite the old files with the new once. Make sure you have a backup prior to that.
2 – What error are you seeing in your dashboard?
It might be better to regain access to your dashboard and then run update from there.

Are you using the default theme or a custom one? Try switching to the default one.

This error is normally caused by an extra space before the <?php tag. Look inside those files and see if you find it.

Do you have a link to that site? What is the exact error that is being displayed?

Did you try this plugin by itself on a clean WordPress install?

Please enable error_reporting mode in PHP. This will at least show you where the issue is.