During signup, our system will look up your existing "MX" records. You will see something like:
| Domain |
Mailserver hostname |
Mailserver IP |
Priority |
| yourbiz.com |
mail.bighost.net |
61.159.200.15 |
10 |
In this example, you must then choose whether to use "mail.bighost.net" (the hostname) or 61.159.200.15 (the IP address). Either will initially work, but there are subtle differences.
If your domain runs at a hosting company, then you are most likely using the hosting company's mail server for your email. In this case, use the "hostname" because hosting companies often change the IP address, but rarely change the hostname.
If your business has its own (in-house) mail server, then the person in charge of it (probably you) may prefer to use the IP address. This will rule out any possible problems resolving the hostname to the proper IP address.
In any case, you can change it later from your Control Panel. Help is available for the Control Panel after logging into it.
If you don't understand this discussion, then simply use the "hostname".