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Mailgun has a whole FAQ type thing about sharing IPs vs getting dedicated. They kind of claim an opposite/divergent thing from what you write. They say using shared as you build domain reputation can help because you pool with more established senders. Did you find that not to be true? Are there many spammers in that pool? Should I just go dedicated right away?

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I find that mailgun is highly protective over thier IP reputation, so unless you actually run into an issue yourself, don't bother with dedicated. Some spammers may use them but I've not seen much evidence of it.