Messages Take a Long Time to Be Delivered When Using Mailgun
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Mailgun often sends messages using the same IP addresses for the pools of multiple clients. These can have SPAM reports filed and other issues.
To find out if this is the reason for the delay, identify the source IP of the message through the Mailgun console:
Next, look up the reputation of that address on any site that provides scores on the sender, such as Sender Score.
When the IP address has a score close to 70%, you can expect to see delays. If it is a paid domain, you should create a ticket with Mailgun support. Ask them for another IP with a score of around 90% to increase the quality of delivery.