Some email libraries will auto-block certain providers based on whether the provider requires verification. This is done in attempt to thwart spam accounts that tend to abuse services. So possibly try a different email provider if you can.

Also, some sites (silently) disallow certain forms of an email address. For instance, I have seen some of my registrations get swallowed for using dev_null+foo@my-domain.com, where +foo is blocked since it can key you in on how that service might use your address (e.g. selling your address to spammers, then +foo would tell you exactly which site sold you out).

I’ve also seen overzealous anti-phishing filters silently reject my registration. For instance, say there’s the site foo.com, I’ll register my email as foo.com@my-domain.com, and I’ll never get the registration email (works on the vast majority of sites, though).

Last one I’ve seen, is old school sites only processing their registrations once per day, usually at night. So possibly wait a day to see if it shows up.