I found that emails sent from projects hosted on WMFLabs are using various ways to get their emails delivered - which results in an extremely inconsistent delivery rate. Besides the
mx-out02.wmflabs.org servers, it seems like tools on the WMFLabs don’t have a unified approach to solve the problem of delivering emails.
It seems like the Wikimedia Space used a trick to let Gmail do all the deliveries (or maybe not, I’m really not an expert on emails), so the recipient’s mailing system will check
gmail.com's SPF record instead of
discuss-space.wmflabs.org. But if it is not this case,
discuss-space.wmflabs.org has no SPF records at all, which will make all emails deemed as spam automatically.
While some other WMFLabs-hosted sites, such as UTRS, the ticketing system for unblock requests on English Wikipedia, will mask sender (bounce address?) as
wmflabs.org without subdomains, thus avoiding the problem that all subdomains of
wmflabs.org have no SPF records. However, I do hear people complaining to me that because their emails from the UTRS system ended up in spam, they missed crucial questions from enwiki admins regarding their requests, thus forgot this matter and have to waste extra time to redo the whole process.
But either way, it seems to me that emails from WMFLabs do seem to have a delivery issue, or at least, not as good as those MediaWiki notification emails from
wikimedia.org. For things like UTRS, email delivery is crucial.
I do hope there could be better ways to improve the system. Not everyone’s using Gmail; there are plenty people using their local service, those provided by their ISP, or regional internet companies like Yandex for Russia, Naver for South Korea, Netease and Tencent for China, etc. I have personally spoken to staff working in Tencent a few years ago to get them to whitelist Wikimedia domains - Tencent’s email takes up probably half of all email usage in China. I don’t think it is difficult to use third-party services like Mailgun for things like the Wikimedia Space, but if that’s not a choice, it will definitely be better to improve the Labs’ email services.