IMAP emails send via cellular data but not via WIFI

[tchara]

Have you checked if it is a similar issue to

Outlook Oauth2 - Cannot connect to server. (Command: NAMESPACE; response: #4# [BAD, User is authenticated but not connected.])

Not necessarily for K-9, but you can set a private DNS that only returns IPv4. Android Settings → Network and Internet → Private DNS Examples (make sure to use the IPv4 address, not the hostname, otherwise they will also resolve AAAA records for IPv6): 5.1.66.255 (Freifunk München) 91.239.100.100 (CensurfriDNS Denmark) 78.46.244.143 (BlahDNS)

or

K9 fetches mail on wifi, but not reliably on cell

If you are on a mobile IPv6 connection, ignore this post… Your mobile network operator may use internal NAT due to lack of IPv4 addresses. OpenVPN maps any port to the VPN connection. Thus, you might end up in a double NAT connection. This always leads to problems. Are you able to activate TURN (Traversal Using Relays around NAT) for your connection?

?