I have a client whose email list has grown to around 400. It may grow to perhaps 1000 one day but is still very tiny!! They send out a newsletter once a month and is done through MSN. This creates a lot of problems as 1) the number of emails that can be sent from MSN are limited to 400 at once and 2) can't use HTML templates. Also the client has to manage all the opt ins and outs but given the small size of the list this is not too much of an issue.
It srtikes me they have two choices. Either move the mailing list off-line and manage it via, say, Outlook Express or use an on-line email management program. Websites such as Aweber.com look great but are very expensive given the very small mailing list.
Try PhpList from (i think) tincan.co.uk or do a search for phpList. It is really slick. Free, no Ads, host it on your own server, configure various templates for each category of newsletters (if more than one).