ForwardMX can be used with any DNS service that allows custom MX records (which is pretty much all of them).
All you have to do is to find the DNS settings, remove any existing MX records and add 2 MX records with the values shown in your account on the "MX Records and Setup Instructions" page.
For example, they might be:
mx1.forwardmx.io - Priority: 10 mx2.forwardmx.io - Priority: 20
You also need to add a TXT record, which is the same for all accounts on ForwardMX:
"v=spf1 include:_spf.forwardmx.io -all"
After that our service can pick up the changes and enable your domain for receiving emails.
These changes are effected automatically as soon as the DNS settings have changed. These takes between minutes and 48 hours.
It is worth noting that it does take only a few minutes with most DNS services, but some email providers may have cached DNS results for up to 24-48 hours and could still deliver emails to the old mail provider.