To increase the reputation and deliverability the SPF record must be configured on the DNS.
The SPF record specified in the domain DNS allows you to indicate which are the IPs designated for sending emails for that domain, this allows the recipient's antispam filters to check that the email does come from the real sender.
This setting is very important to avoid spoofing, i.e. someone trying to send emails on your behalf.
Configure the SPF as follow :
Hostname = domainname.ext. (the final dot is sometimes important)
Text = "v=spf1 +a +mx include:spf.utixo.net ~all"
TTL = 3600
This authorizes the sending of emails from the site hosting server, the receiving gateway and the pool of IP addresses specified in spf.utixo.net
Once the record has been set it is useful check that it is recognized there are many online services to do that.
the -all parameter indicates HARD-FAIL, it forces the recipient to accept emails only from designated SMTP servers.