I tried mailjet out starting with a free tier account. All my emails are transactional emails. Everything was pretty good. For the second month, I upgraded my account to a paid tier ($16/month) because I anticipated higher email volume. Everything was still good. After the second month, I downgraded my account back to the free tier due to lower email volume. This was when everything went unstable. Emails were missing delivery randomly. Some emails were delayed quite a bit. Sometimes the API only responded to repeated requests. This instability was correlated to downgrading my account. My plan was upgrading and downgrading my account based on my needs. Apparently this pushed their wrong button. I contacted them twice, once through email, once through feedback link. No response. Below was my exact message to them. Mailjet, don't bother to ask me to contact you to work anything out. I tried, I gave up and moved to another provider. I am happy now.
"I adjusted my account based on expected email volume. The downgrade was due to the expected volume in the near future. When volume goes up, I will upgrade again. HOWEVER, since the downgrade, I noticed marked degradation in performance. Some emails didn't get sent, sometimes the API was not responsive. I didn't see that before the downgrade. Can you provide an answer?"