50k is a lot of emails. That said, I have maintained a couple of email lists in the 1500-5000 range. Used DadaMail on a VPS. Took about an hour for the larger list to process.
Be advised, if you are hitting major ISP's or net mail with a ton of emails (think Y-Mail, G-mail, AOL, Bellsouth, etc.) you may well get flagged as a spammer - whether guilty or not.