Nothing high tech. Started out monitoring failed deliveries and sent out verification requests manually to get over the first week or two. I noticed it was failing consistently on Microsoft domains, so contacted the provider and they reassigned an IP which improved things. Other than that, I have a really high open rate, but I'm not going to pretend I know much about the intricacies of building a domain's reputation. I'm using Mailgun btw. Can't really compare it to other services personally, but their support has been great especially given that my service isn't large enough yet to need more than the free tier.