Bulk email service using an API

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We are web development company looking for bulk e-mail services using
an API.  We have a contact manager website for one of our clients and
are looking to expand the tools on the site to include e-mail blasts.
We want to subcontract a company that can send and provide success
rate information about the email blasts to each registered customer,
without leaving the user interface.   We have over 100 users (each of
whom will be sending individual e-mails per month) and approximately
100,000 e-mail per month in total.

Has anyone worked with a good company offering competitive pricing and
quality technical support for API e-mail blasts?
Companies we=92ve briefly looked into: StreamSend, FuseMail, EmailLabs,

Thanking you in advance.

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