The rationale seems to be that shared IPs are cheaper for them (and the customer). It's also possible that Postmark doesn't have the capacity to operate a large number of dedicated IPs for individual customers.
Any large-scale sender for whom deliverability is critical should be using a dedicated IP. By virtue of hosting multiple customers, shared IPs appear to send more frequently, and any behavior on a shared IP resulting in a blacklist entry affects everyone else on that IP.
We're pretty aware of this problem at SendGrid. Even though we make every effort to cull bad senders from our shared pools, our enterprise senders nearly always prefer dedicated IPs, which let them build up a trusted reputation without interference from others.
If we only send a few hundred emails a month, but need everyone to reach customer's mailbox (transactional emails, not marketing emails), would you recommend a dedicated IP? how would I know if the dedicated IP has ever been/is blacklisted too?
We're using Mandrill, but we am looking at alternatives longer term.
If you're only sending a small amount of desirable transactional email, you're the perfect model of a good sender and your blacklist risk will be vanishingly low. Sending from a shared IP will still expose you to the behavior of the other customers using it (like a VPS, you'll have neighbors). We actively police our shared IPs for bad sender behavior.
Dedicated IP packages won't be affected by the sending habits of other users. They are more cost-efficient for high-volume senders, but we do have dedicated IP tiers starting at the $80/mo. mark, which allows up to 100k mails a month.
With either option, we're always happy to work with you if you're having deliverability issues - and that includes checking IPs for blacklist status.
Any large-scale sender for whom deliverability is critical should be using a dedicated IP. By virtue of hosting multiple customers, shared IPs appear to send more frequently, and any behavior on a shared IP resulting in a blacklist entry affects everyone else on that IP.
We're pretty aware of this problem at SendGrid. Even though we make every effort to cull bad senders from our shared pools, our enterprise senders nearly always prefer dedicated IPs, which let them build up a trusted reputation without interference from others.