I've run into this issue a number of times. Our DNS settings were all good (DKIM and SPF confirmed my Mandrill) and after some investigation (looking at the headers of the delayed emails) the delay appeared to be entirely on Mandrill's side (once it was handed off to Gmail or Yahoo the delivery occurred within a second). When I contact Mandrill support they explained why we were seeing these delays:
In looking over the logs for your account we are seeing intermittent
delays for some of your recipients. Generally, the speed of delivery
in most cases depends largely on the receiving domain, and how quickly
they will receive and process emails. Most of the major email
providers limit how much email they'll receive in a certain period of
time, and will restrict delivery—Mandrill's sending servers are
designed to queue and back off sending if this occurs. In these cases,
the receiving mail server or ISP will return a specific kind of SMTP
response telling Mandrill's servers to 'back off' and 'try again
later,' which ultimately results in the message lingering on our mail
servers longer than expected (and since the message isn't passed off
to the receiving server at that point, and we're only getting a 'try
again' response, you won't see that information in the message headers
of the final email you receive. You'll only see that the email stayed
on our servers for a longer time period which can be confusing).
Additionally, even though we may hand the messages off to ISPs for
delivery almost immediately, it's still up to that ISP, like Gmail or
Yahoo, to actually to process that email and place it in the inbox.
Each receiving server is different though, so it may take a different
amount of time for Yahoo to process the mail than Gmail, for example.
In many cases, things like the time of day and overall email traffic
to that recipient server can affect how quickly they're able to
receive and process email.
All that said, the delays you're seeing generally aren't expected, and
while we see that messages are ultimately delivering, we are detecting
factors on our end where we may need to make some changes to help
mitigate further delays. Our delivery team is continuing to monitor
traffic to major ISPs and will make necessary adjustments as needed.
We still periodically see these delays, though they've improved is so the delays are rarely longer than 10 minutes or so, but it still can cause issues with things like password resets or confirmations that are time-sensitive. Bottom line: Mandrill is awesome for bulk mailing, but if you need instantaneous delivery you may want to rely on a different or self-hosted service.
