We have a dedicated mail-server built into our VPS that handles all outgoing forum messages. That hasn't changed since we moved over to XenForo, which is when this problem started.
I'll double check. Like I said before, this wasn't an issue until we switched over to XenForo, and Gmail is...
I've had multiple users inform me that their gmail (and other email providers) have been flagging the content of xenforo password resets and validations as spam due to being "similar to other spam messages". This is not an issued that we had when we were using vBulletin, so I know our domain has...
Judging by the tone of your post, you don't have a lot of experience with server/VPS administration. I would suggest learning about it now and practicing your skills building stacks BEFORE you think about jumping into managed/un-managed VPS. It's not easy, but it's not impossible.
Meanwhile...
Is there a quick SQL query or adminCP option that would allow me to automatically pull and set gravatars for all users who have a gravatar account? I've tried using the search function, but everyone seems to want this feature disabled...