how not to cascade through MX records

[stfaprc]

One of our customers has an email system that only tries the first MX record for our domain and does not try any other of the MX records if the first one fails.

What do I tell them to change so that the email server will not stop at the first MX but go down the list of MX records?

 

[elgrandeperro]

Are you setting different MX precedence values? Or do you have servers at the same level?

 

[stfaprc]

Different levels, 5 and 15

we only have this one customer who won't cascade through the MX servers.