Once you've done that you should look at what other services you rely significantly upon and see about mitigating the risk there by having an alternative ready to go (or even sharing the load).
At a previous startup we realised we relied wholly upon Mandrill and so reimplemented the sending code so that half of the emails went out via Mandrill and the other half by SendGrid. A stunt like the above just requires a quick reconfigure to make all emails go via the alternative provider whilst we (with less panic) add another new alternative provider to share the load. It also helps build up a positive reputation before cutting over straight away.
(This wasn't about splitting the emails amongst free tiers to keep it free, we were far away from moving up to a paid tier even with all emails going through one provider.)