Uploading your contacts from your computer or company database can be problematic. Depending on how long you have been adding contacts to your database there is a good chance a high percentage of your emails are no long valid. This will cause a high bounce rate and could get your suspended from the bulk email giants.
It’s a good idea to have a 3rd party clean your list before uploading it for use. The company I choose, with great reviews, was Voilanorbert.com Their pricing was very reasonable and found that 35% of my list had invalid email addresses 39% were risky and only 25% were clean, usable email addresses. Mind you, this list went back over 15 years. For checking ~1400 email addresses the cost was $4. Very reasonable. I could have paid an additional $50 and they would have filled in my list with missing names, titles, companies, addresses & phone number.