Problem sending emails using large group

We have a client who has a customer base of 5k plus. On our old server he routinely sent this group email and got no errors. On our Nexcess VPS, he is advising that about half of the messages get sent and then the connection fails. Does anyone know of a setting that might limit the email number? My first step will be to increase the timeout but I wanted to post here, as well, to check on other possible causes.

Thank you,