Cannot send or receive email ('Message not sent. Server replied:)

[joximu]

Some tipps, but they might not be the problem here:

take care about the gid and uid for user vmail in main.cf.
Don't use values above 2000 - it mayy lead to problems with ispCP when creating the 8th customer...

$myhost is not a fqdn,
$mydomain: you don't want to use this domain name as a customer domain name?

but this has nothing to do with the SASL thing...