[exim] Strange log entry about backup MX

Hi,

we have the following log entry about the backup MX:

450 4.1.0 Don't use the Backup MX 'relay.rzone.de' while the Primary
MX is available - please send your mail to ...

Does exim use the Backup MX if the primary MX is available? Can I
configure exim to prefer the primary MX?

Thanks for any hints.

tilo


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