example.com. IN TXT v=spf1 mx -all
What does this mean?
If the IPAD-OS is your email server and you don't have anyone doing
secondary mail service for your domain, answer YES to this question.
If your ISP or someone else is providing secondary inbound mail service, sometimes called secondary MX, and they allow you to relay mail from for your domain out through their server to other places on the Internet, answer YES here.
For everything else, answer NO here.
By the way, spammers love domains that use more than one (a.k.a. secondary) MX records because it is much harder for you to keep the same level of spam fighting ability on all of the mail servers that accept mail for your domain. Whenever possible it is best to not have more than one MX record for your domain. This will help limit SPAM in your mailbox.