We're running into some trouble with our email being marked as spam. We are looking for a web service to provide the solution.
It seems that services like MailChimp & Campaign Monitor focus primarily on sending large amounts of similar marketing emails. The nature of our emails is different however: it's only being sent to people that we know personally and are paying us. An "unsubscribe" link is not applicable either, since there is no "list", the emails are sent on-demand by users.
Also, they seem to be focussed on marketing/design. We have the developers, we just want to outsource the delivery problem.
In addition, we have very few emails. However, they may contain sensitive data (password resets, invoices) and may be quite large per email (in the case of invoices). Being able to send attachments is also a requirement.
Can you recommend a service that fits these needs?
EDIT: added "web service" as per comments.