SMTP issues using 1.1.1.1 as main resolver

Hello.
I’m encountering various SMTP issues using 1.1.1.1 as resolver on an email server.
In /etc/resolv.conf I have

# Generated by NetworkManager
nameserver 1.1.1.1
nameserver 8.8.8.8

and this brings to very long SMTP delays (up to 25’ to send a single message, both by webmail or external client)

Just changing it to

# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 8.8.4.4
nameserver 1.1.1.1

everything works fine, and sending is almost immediate.

Anyone having the same issue with Cloudflare’s DNS ?

Regardless of whether you (or cloudflare) may have a problem, I would strongly advise you NOT to use a public resolver for a mail server.

Best is a local recursive resolver (unbound is a common choice). Otherwise you will have problems using various black/white-listing services, which rate limit (or even block) if they are abused (in their terms, and they get to chose them!) by being accessed from a single client (from their point of view, cloudflare, or google, or whatever).

In short: use your own resolver.