If you do change host/ISPs, here are some things to consider.  Some additional issues relate to virtual hosting, DNS configs, whitelisting that your customers may need to do.

/Pete

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Off the top of my head (and in no particular order):

Questions (some may overlap):

how many customers do they have
what percentage are paying customers
how many lists are hosted
how many subscriptions do they have on all lists
who is their backup / disaster recovery providor
what are their hours of customer service
how long have they been in business
do have have a UPS and generator
how long can they operate
what are they doing to filter spam and viruses
what are their rates for service
what forums do they provide for customer help
what is their service level agreement [SLA]
what is the contract period
what are the separate roles customers can delegate e.g., owner, moderator
what kinds of list administrative functions do they provide for error / bounce / subscription handling
have they produced any whitepapers, best practices documents, or case studies
how scalable is their solution
how secure is their installation (do they have active intrusion detection)
how reliable is their service
what interface does one user to maintain the list
what interface does subscriber use to self-maintain their options
do they support moderated broadcast lists like TVC-Alert
can any one of several moderators approve a message
what method do they use to prevent spoofing so that only authorized posters may transmit
who are their closest competitors
what is their thruput rate for mail delivery
how do they handle blocked (client) mail servers
what services do they offer to customize mailings
do they have any "measurement" services for determining email response rates
what do they do "better" than L-Soft's LISTSERV(R) solutions?
what are their confirmed opt-in procedures that prevent subscription spoofing

Just FYI:

http://www.google.com/search?num=100&hl=en&safe=off&q=%22mailing+list+software%22+features&btnG=Search

http://www.lsoft.com/resources/wp-licensing.pdf

http://www.lsoft.com/resources/wp-emaillistintro.pdf

http://www.lsoft.com/resources/wp-emaildeliverability.pdf

(c) 2007 Peter M. Weiss