Gmail will stop accepting your emails if they are not DKIM-signed or without an SPF record. Here is a detailed article on this issue from Spamresource.com.

What you should do

• Check your emails for DKIM and SPF records.
• Check your nuevoMailer activity log for errors.
• Check your email clients report. If you had seen a drop in Gmail then something may be wrong.
• Use Google's postmaster tools.
• With nuevoMailer SB v.9.9 and ESP v.4 you can DKIM-sign your emails. You should only do this if your provider does not give you this option. You can read more on this here.

The nuevoMailer API and Zapier app are now available also for your clients.

Furthermore, they have Subscriber tags and Outbound webhooks. See all new features in nuevoMailer ESP v.4 here.