Repeating a suggestion I made by email about 3 years ago, in case it might be feasible in the latest Mercury ...
It would be really handy to be able to specify a fall-back SMTP server address or addresses in MercuryE. It doesn't have to be anything particularly fancy - simply add the specified address(es) to the end of the list of MX addresses for each outgoing message, so that if all the MXs are unavailable or refuse the message (a common problem with blanket blacklisting of private IP addresses by some large ISPs), then the message would be sent instead via (say) my ISP's mail relay.