After the demise of Mandrill I started to divide transactional email between Elastic Email and SparkPost. I send around a thousand messages a month through them and haven't had any problems with either. I still miss Mandrill though. I send high volume mail through Mailgun and have had a great experience with them and their API. I tried integrating with Sendgrid but they have some constraints around MX records that were showstoppers for me.