Vertical Response is offering a white paper called "Eight things to consider when choosing an ESP". The list of criteria they offer is quite comprehensive, but in my opinion, it misses one important criteria: if you will use the ESP to send emails in multiple languages you should test the capabilities of the ESP in these different languages.
This is a piece of advise I'm offering from my own personal experience working with a US-based ESP that did not support special characters for any of the European languages like umlauts, accents etc... The workaround was extremely labout intensive, so please make sure to test this if it applies to you!
PS. Vertical Response are an email service provider theirselves. If you're looking for a more objective source or you want to compare vendors against each other, you might want to have a look here:
- MarketingProfs' Vendor Selector for Email Service Providers
- JupiterResearch's E-mail Service Provider Market Guide, 2005
- E-consultancy's Email Marketing Buyer's Guide 2005 (UK)
- Choosing The Right Email Service Provider For Your Campaigns
Most of these sources focus on the US market. I don't know if there is a source for European vendors, but I would be very interested to find out. If you happen to know one, please let me know!