Hey, I found something new

If your mail is not delivered to gmail, you need to check if your mail provider has an SPF OR DKIM record set up in you mail account.

If not, e-mails are getting rejected by gmail.

https://support.google.com/mail/answer/81126?hl=en&sjid=6130548297486259015-EU#authentication

I just tested it and if you use gmail for registration, it goes into spam folder every time

Seems to be a problem with trustworthiness of your provider you sending confirmation mails from

You may get a email soon from Gmail and Yahoo, sometimes it can even take even a day or two, it will give you a code telling you what's up and why mail was not delivered, also try to also always ask for a mail delivery reply, you can set this in your client, usually. Not sure how OSSN handles things but yes as Arsalan said make sure the domain your using is tickety boo with sending email, meaning, meets all sending requirement, you can test things at mxtoolbox for eg.

Gmail, Yahoo and Microsoft are very strict because of spam etc which is a bit of a joke as it was their email services that had a big part in the sending of spam in the first place when the Internet really started to take off and still is.
A pet hate of mine.

Then again it can be a plethora of other reasons.

Seek and you will find the answer.

Make sure your IP is not black listed. If emails are going to outlook then then make sure you have proper spf, dkim, dmarc. mx entries assigned.