No error messages, but the email doesn't arrive

Symptoms
After submit the contact form, on screen notification informs you that the email was sent, but it never reaches your mailbox.
It is rejected and returned to the sender, or goes right into the spam folder.

Requirements
This document assumes you are familiar with

1. Sender Policy Framework (SPF)
2. Deploying / Publishing SPF Records
3. Test SPF records

Since these are technical changes that could impact on your entire domain, and details of these changes vary by system, we strongly recommend they be made with the help of a qualified system administrator, who will be perfectly able to identify the actions to be taken.

Solution
SPF problems are related to the anti-spam policy of the provider who handle the mailbox where you aspect to receive the submissions, but it's caused by a bad DNS configuration in the domain to which your server belongs, and note that this domain is not necessarily your domain name. Often it is your provider's domain name indeed.
You have to analyse the headers and the route of outgoing emails, and set the SPF DNS records accordingly.
A few examples follows, just to give to the outsiders, an idea of how SPF works.

Example of a failing delivery

1. Joomla and Fox Contact are installed at the URL "http://yoursite.com".
2. The domain "yoursite.com" is hosted by "yourprovider.com", an Internet Service Provider, on a machine called "hostname", with ip address "1.2.3.4".
3. Fox Contact is configured to send email notifications to youraddress@gmail.com
4. You submit a contact form.
5. The email is automatically marked with:
   • Return-Path: <yourftpusername@hostname.yourprovider.com>
   • Sender: <yourftpusername@hostname.yourprovider.com>
6. The email then arrives to the GMail server.
7. GMail server reads the SPF records by the DNS. Note that it doesn't read SPF records for "yourdomain.com", but for "yourprovider.com", because of Return-Path and Sender headers. SPF records say that only IP address 1.1.1.1 is designated to send email from "yourprovider.com", but the email arrived from ip "1.2.3.4", so it is rejected and returned to the sender.
8. The sender is yourftpusername@hostname.yourprovider.com. Probably this mailbox doesn't exist, so the email will be permanently lost.

Example of a succesfully delivery

1. Joomla and Fox Contact are installed at the URL "http://yoursite.com".
2. The domain "yoursite.com" is hosted by "yourprovider.com", on a machine called "hostname", with ip address "81.31.144.124".
3. Fox Contact is configured to send email notifications to youraddress@gmail.com
4. You submit a contact form.
5. The email is automatically marked with:
   • Return-Path: <yourftpusername@hostname.yourprovider.com>
   • Sender: <yourftpusername@hostname.yourprovider.com>
6. The email then arrives to the GMail server.
7. GMail server reads the SPF records for "yourprovider.com", then it accepts the email and it adds the following headers:
8. Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of yourftpusername@hostname.yourprovider.com designates 81.31.144.124 as permitted sender)

Armindo,
your website radioliberdadedili.com is hosted by rumahweb.com on the machine rebellion.rumahweb.com with IP address 50.22.122.2.
The email generated is marked with:
Return-Path: <radio126@rebellion.rumahweb.com>
The email recipient is m#####@radioliberdadedili.com
Let's see who handle the mailboxes for radioliberdadedili.com...
radioliberdadedili.com mail is handled by 0 radioliberdadedili.com.
radioliberdadedili.com mail is handled by 0 ASPMX.L.GOOGLE.com.
Now let's check SPF records for rumahweb.com :
only 173.192.123.180 and 173.192.107.82 IP address are acceptable, but your mail was originated from 50.22.122.2, so your ASPMX.L.GOOGLE.com server will reject it.

Your provider has a bad DNS configuration.

Please post your Joomla Mail settings and the email address you want to use to receive messages.