It's not the MTAs that don't like Qmail, it would (my hypothesis) be the scoring systems run by the ISPs that would (hopefully automatically, i.e. based on stat

It's not the MTAs that don't like Qmail, it would (my hypothesis) be the scoring systems run by the ISPs that would (hopefully automatically, i.e. based on statistics) give senders using some MTAs a lower score than others. Small users generally don't use Qmail anymore, so if you're a small sender and use Qmail, then perhaps that's odd, and iff spammers (perhaps for Qmail's efficiency (1)) are using Qmail often, then... (1) although you can attain higher efficiency with a custom MTA that's not safe, i.e. big spammers generally don't even retry for efficiency (that's why greylisting works). A friend of mine runs several mail servers on OpenBSD, using OpenSMTPD, and says he's got almost zero problems and scratches his head on what I'm doing. So there's 3 hypothetical reasons for that for me: - basically no spammers will be using OpenBSD (why should they?), - and hence also none be using OpenSMTPD, - and hence also none of them will be using a OpenBSD hoster (netblock!). Relatively many spammers (perhaps not the biggest ones, but still spammy senders) will be using services like Hetzner, and will most likely be running Linux. Now if Gmail etc. do check the MTA you're running, and perhaps do OS fingerprinting, they'll "know" how to judge you. Anyway, I don't have evidence that Qmail is a big reason, it seems likely the netblock is much more important (or at least that's what I'm inclined to think because the alternative would indeed be rather frustrating).