I use mailgun to send automated alerts from our home network backup processes and security cameras.
When sending to a Gmail account, there is sometimes up to a 2 hour delay before the mail arrives, though usually it's within a few minutes.
Mail sent to a Fastmail account always arrives almost immediately.
MG does some kind of prioritizing based on the domain when using their free account. It's obviously an anti-spam measure. They might also have some kind of "warm-up" feature where if you send enough email with a valid FROM and TO address then they queue it faster, IDK. Not sure if they do the same thing with their paid plans.
My understanding is that they rate limit the number of emails sent to a domain per IP address, per hour. Email service providers assume that if you send "too many" emails to their domain per hour, it's "spam".
The same thing happens with my dedicated IPs (sending ~500k/mo). Yahoo is frequently rate limited. Gmail is less common, though we have years of good reputation on those IPs.
Our product is a SaaS for pet service businesses (grooming, dog daycare/boarding, training). The app has 34 distinct automated email types that are sent at various points. It's a lot of email :).
When sending to a Gmail account, there is sometimes up to a 2 hour delay before the mail arrives, though usually it's within a few minutes.
Mail sent to a Fastmail account always arrives almost immediately.
MG does some kind of prioritizing based on the domain when using their free account. It's obviously an anti-spam measure. They might also have some kind of "warm-up" feature where if you send enough email with a valid FROM and TO address then they queue it faster, IDK. Not sure if they do the same thing with their paid plans.