1) check your junk. 2) if it's gmail, that's why I don't use gmail, they actively BLOCK all my legitimate traffic whilst letting spam through; the opposite of what everyone else claims it does. 3) See the BIG YELLOW BOX on the page with instructions of what to do if you don't get your activation mail? Yeah that. -- edit -- I just double checked and it IS sending registration e-mails to gmail properly, so not sure why you would not have received yours. It's not even spam boxing them anymore. Anyhow, you're "known enough" here I went ahead and did a manual activation.
It's not SMF's fault so much as my hosting and Gmail. For some screwed up reason every five minutes gmail finds some new lame excuse to reject mails from my server. It's been an endless case of whack-a-dhole. They bounce so hard they don't even reach people's spam boxes. ... and all I get on my end in the server logs is "rejected" with no other data. Yet strangely they DO go through for SOME people, myself included. There's no rhymne nor reason to it. ... and of course much like if you have adsense issues, you TRY to find where to ask Google for help, FINALLY find where they buried it, and their response is "let them eat static". Helpful. Much like everything Apple, or front-end frameworks, or dozens of other things, I cannot fathom why anyone uses gmail by choice. NEVER been useful for me, ALWAYS been broken for me... and if I use it at all, it's as a black-hole address.
How did you fix the gmail registration issue. On my SMF forum People who register with gmail email never receive the verification email.. However all other types of emails work.
Gmail is VERY picky. The things they "want" 1) Your PTR record for the server must be correct. Whatever domain the server thinks it is when you set it up? Your PTR record must point at that so RDNS works. This isn't too much of a surprise, I use RDNS in my contact forms to make sure spammer aren't bullshitting domain names. 2) You need to make sure that spf records are included on your DNS. Basically for both "@" and "mail." you want to make sure a TXT record "v=spf1 a ~all" is set up. Thankfully I'm using namecheap to manage DNS instead of my own server, so it's a lot easier to configure there. 3) setup and install DKIM. This is a pain to explain... But Linuxbabe's tutorial is pretty good on debian-likes. https://www.linuxbabe.com/mail-server/setting-up-dkim-and-spf Laugh is given what's actually plugged into these systems and how they work, it's ALL placebo bullshit in the war against spammers, but Google has a decade+ love affair with placebo BS.