As long you have the email reach your inbox. That should be OK.
From what I am experience with, the time it takes to deliver is different occasionally. Sometime I got email so fast, sometime it is a little bit longer. And depend on so many things such as SpamAssasin gate-way, server in the same region, your VPS hosting services, etc…
Often Gmail, Outlook.com, Yahoo.com have setup a strict rule for filtering spam emails. It takes time to build up your email domain reputation. There are lots of techniques surrounding it. Just google and you will find tons of them.
My best tips is sending to the one have issue with spam box, mark it as not spam, reply to your MIAB email domain, do it more times. Might send a request to the email provider and explain why it lands on their spam box.
Last thing also very important: “CHECK YOUR BLACKLIST IP ADDRESS SERVER” at Mxtoolbox.com
Sometime I have to reinstall the whole MIAB server just because I been given a BLACKLISTED IP address from the VPS provider.
Hope it helps
As Quang said, you should always check the IP against blacklists when you spin up a VPS to use for mail, before you go through the process of setting things up. If it shows on any of the lists, just destroy it and create a new one.
For some reason, I have also noticed that when we create servers on Digital Ocean (on non-blacklisted IPs), we seem to end up with a lot more stuff going into Gmail and Outlook spam than with a VPS on any of the other big cloud providers.
One particular person/client I work with I keep getting this -
This is the mail system at host (my domain) I’m sorry to have to inform you that your message could not be delivered to one or more recipients. It’s attached below. For further assistance, please send mail to postmaster. If you do so, please include this problem report. A URL in this email (my domain) is listed on https://spamrl.com/. Please resolve and retry (in reply to end of DATA command)
had to put my domain because of restriction number of links in a post
Certain large providers who think that they own the internet (I wont mention names) blacklist IP ranges belonging to Digital Ocean. This is an internal blacklist, not one you will find using MXToolbox or similar tools.