I can confirm this problem.  'dig' works because by default it's only
asking for A records; but applications on ipv6-enabled clients will ask
for both A and AAAA records, and if I query AAAA for this name, the
response is too big to fit in a udp packet:

$ nslookup -q=aaaa pod51041.outlook.com 192.168.15.1
;; Truncated, retrying in TCP mode.
Server:         192.168.15.1
Address:        192.168.15.1#53

Non-authoritative answer:
pod51041.outlook.com    has AAAA address 2603:1036:d02::2
pod51041.outlook.com    has AAAA address 2603:1036:d02:6::2
pod51041.outlook.com    has AAAA address 2603:1036:d02:7::2
pod51041.outlook.com    has AAAA address 2a01:111:f400:5201::2
pod51041.outlook.com    has AAAA address 2a01:111:f400:f370::2
pod51041.outlook.com    has AAAA address 2603:1036:3:cc::2
pod51041.outlook.com    has AAAA address 2603:1036:3:108::2
pod51041.outlook.com    has AAAA address 2603:1036:4:6f::2
pod51041.outlook.com    has AAAA address 2603:1036:4:71::2
pod51041.outlook.com    has AAAA address 2603:1036:101:3a::2
pod51041.outlook.com    has AAAA address 2603:1036:102:53::2
pod51041.outlook.com    has AAAA address 2603:1036:102:cb::2
pod51041.outlook.com    has AAAA address 2603:1036:405:3b::2
pod51041.outlook.com    has AAAA address 2603:1036:804:1::2
pod51041.outlook.com    has AAAA address 2603:1036:804:a::2
pod51041.outlook.com    has AAAA address 2603:1036:902:a3::2
pod51041.outlook.com    has AAAA address 2603:1036:906:4f::2
pod51041.outlook.com    has AAAA address 2603:1036:d01:1::2

Authoritative answers can be found from:
outlook.com     nameserver = ns2.msft.net.
outlook.com     nameserver = ns3.msft.net.
outlook.com     nameserver = ns1.msft.net.
outlook.com     nameserver = ns2a.o365filtering.com.
outlook.com     nameserver = ns4.msft.net.
outlook.com     nameserver = ns1a.o365filtering.com.
outlook.com     nameserver = ns4a.o365filtering.com.
ns1.msft.net    internet address = 208.84.0.53
ns1.msft.net    has AAAA address 2620:0:30::53
ns2.msft.net    internet address = 208.84.2.53
ns2.msft.net    has AAAA address 2620:0:32::53
ns3.msft.net    internet address = 193.221.113.53
ns3.msft.net    has AAAA address 2620:0:34::53
ns4.msft.net    internet address = 208.76.45.53
ns4.msft.net    has AAAA address 2620:0:37::53
ns1a.o365filtering.com  internet address = 157.56.110.11
ns2a.o365filtering.com  internet address = 157.56.116.52
ns4a.o365filtering.com  internet address = 157.55.133.11

$

If I try this against systemd-resolved, I see:

$ nslookup -q=aaaa pod51041.outlook.com 
;; Warning: Message parser reports malformed message packet.
;; Truncated, retrying in TCP mode.
;; Connection to 127.0.0.53#53(127.0.0.53) for pod51041.outlook.com failed: 
connection refused.

$

So the problem is that systemd-resolved is not handling tcp requests at
all.

** Changed in: systemd (Ubuntu)
   Importance: Undecided => High

** Changed in: systemd (Ubuntu)
       Status: Confirmed => Triaged

** Summary changed:

- systemd-resolved fails to fall back to TCP for large records (Cannot ping 
pod51041.outlook.com but can dig.)
+ systemd-resolved does not listen on TCP port, cannot serve large records 
(Cannot ping pod51041.outlook.com but can dig.)

-- 
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https://bugs.launchpad.net/bugs/1731522

Title:
  systemd-resolved does not listen on TCP port, cannot serve large
  records (Cannot ping pod51041.outlook.com but can dig.)

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