I changed from Eircom to Utv last year (still waiting for land line to switch) and kept my eircom address. Incoming mail server POP3 is still the same - mail2.eircom.net.
Was told by Utv to change outgoing mail server to mail.utvinternet.net. It works perfectly. As you probably know, just go to Tools in OE, Accounts, Mail, Properties then Servers. Don't know how or if it would work if the outgoing server wasn't changed to Utv. AFAIK nothing would need to change for NTL since they don't use the phone line.