by williamconley » Thu Dec 12, 2019 6:43 pm
Mai transport is based on postfix configuration if you are using postfix for delivery. But it is also dependent upon your route. If you are going through a system with a very limited bandwidth for email, a WAV file can take an inordinate amount of time to traverse to the destination and some intermediaries will limit the bandwidth on purpose to discourage abuse and reduce the impact on their pass-through systems. Plus, the reputation/configuration of mail permissions for the domains involved in the mail packge make a difference. If you are sending as "bob@emailmonkey.com" and your server's IP does not have explicit permission to send (Google "SPF email") email on behalf of emailmonkey.com, there can be delay or even (or eventually) a rejection based on that lack of permission. If you are going through a major email system such as google, they will often delay everything to see if you are about to spam everyone. You don't spam? they release after ... an hour or so.
You can set postfix to offload the email to a major mail relay (gmail, live, whoever) by using credentials and an actual mail account on that service to dramatically increase their belief that you are not spamming, but that may not remove any bandwidth chokes from ISPs in the way of the transport.
So: What transport are you using? What email path is in use?
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