Just wondering what the domain is and what settings you are using?
with my isp the pop server is mail.guam.net using port 995
The smtp server is smtp1.guam.net using port 465
I use the linux stunnel to tunnel the ssl from Pegasus  with settings below.
Have similar option for gmail access. 
[guampop]
client=yes
accept = 127.0.0.1:20996
connect = mail.guam.net:995
debug = 7
[guamsmtp]
client=yes
accept = 127.0.0.1:20466
connect = smtp1.guam.net:465
debug = 7
would run dig command if you have it locally or web base ones.
on linux 
dig -tany gmail.com 
ouput:
; <<>> DiG 9.18.17 <<>> -tany gmail.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28694
;; flags: qr rd ra; QUERY: 1, ANSWER: 11, AUTHORITY: 0, ADDITIONAL: 9
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;gmail.com.            IN    ANY
;; ANSWER SECTION:
gmail.com.        85934    IN    CAA    0 issue "pki.goog"
gmail.com.        3134    IN    MX    20 alt2.gmail-smtp-in.l.google.com.
gmail.com.        3134    IN    MX    5 gmail-smtp-in.l.google.com.
gmail.com.        3134    IN    MX    40 alt4.gmail-smtp-in.l.google.com.
gmail.com.        3134    IN    MX    10 alt1.gmail-smtp-in.l.google.com.
gmail.com.        3134    IN    MX    30 alt3.gmail-smtp-in.l.google.com.
gmail.com.        32    IN    A    142.250.188.229
gmail.com.        15099    IN    NS    ns3.google.com.
gmail.com.        15099    IN    NS    ns4.google.com.
gmail.com.        15099    IN    NS    ns1.google.com.
gmail.com.        15099    IN    NS    ns2.google.com.
;; ADDITIONAL SECTION:
ns1.google.com.        16199    IN    A    216.239.32.10
ns2.google.com.        16867    IN    A    216.239.34.10
ns3.google.com.        16867    IN    A    216.239.36.10
ns4.google.com.        16867    IN    A    216.239.38.10
ns1.google.com.        102407    IN    AAAA    2001:4860:4802:32::a
ns2.google.com.        119474    IN    AAAA    2001:4860:4802:34::a
ns3.google.com.        15099    IN    AAAA    2001:4860:4802:36::a
ns4.google.com.        15099    IN    AAAA    2001:4860:4802:38::a
;; Query time: 16 msec
;; SERVER: 127.0.0.53#53(127.0.0.53) (TCP)
;; WHEN: Tue Aug 22 02:44:23 ChST 2023
;; MSG SIZE  rcvd: 452
MX records are mail exchange, but doesn't list the pop3 server??
Be interesting if you check domain and find they do have a pop3 server running.
Just wondering what the domain is and what settings you are using?
with my isp the pop server is mail.guam.net using port 995
The smtp server is smtp1.guam.net using port 465
I use the linux stunnel to tunnel the ssl from Pegasus  with settings below.
Have similar option for gmail access. 
```` 
[guampop]
client=yes
accept = 127.0.0.1:20996
connect = mail.guam.net:995
debug = 7
[guamsmtp]
client=yes
accept = 127.0.0.1:20466
connect = smtp1.guam.net:465
debug = 7
````
would run dig command if you have it locally or web base ones.
on linux 
dig -tany gmail.com 
ouput:
```` 
; <<>> DiG 9.18.17 <<>> -tany gmail.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28694
;; flags: qr rd ra; QUERY: 1, ANSWER: 11, AUTHORITY: 0, ADDITIONAL: 9
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;gmail.com.			IN	ANY
;; ANSWER SECTION:
gmail.com.		85934	IN	CAA	0 issue "pki.goog"
gmail.com.		3134	IN	MX	20 alt2.gmail-smtp-in.l.google.com.
gmail.com.		3134	IN	MX	5 gmail-smtp-in.l.google.com.
gmail.com.		3134	IN	MX	40 alt4.gmail-smtp-in.l.google.com.
gmail.com.		3134	IN	MX	10 alt1.gmail-smtp-in.l.google.com.
gmail.com.		3134	IN	MX	30 alt3.gmail-smtp-in.l.google.com.
gmail.com.		32	IN	A	142.250.188.229
gmail.com.		15099	IN	NS	ns3.google.com.
gmail.com.		15099	IN	NS	ns4.google.com.
gmail.com.		15099	IN	NS	ns1.google.com.
gmail.com.		15099	IN	NS	ns2.google.com.
;; ADDITIONAL SECTION:
ns1.google.com.		16199	IN	A	216.239.32.10
ns2.google.com.		16867	IN	A	216.239.34.10
ns3.google.com.		16867	IN	A	216.239.36.10
ns4.google.com.		16867	IN	A	216.239.38.10
ns1.google.com.		102407	IN	AAAA	2001:4860:4802:32::a
ns2.google.com.		119474	IN	AAAA	2001:4860:4802:34::a
ns3.google.com.		15099	IN	AAAA	2001:4860:4802:36::a
ns4.google.com.		15099	IN	AAAA	2001:4860:4802:38::a
;; Query time: 16 msec
;; SERVER: 127.0.0.53#53(127.0.0.53) (TCP)
;; WHEN: Tue Aug 22 02:44:23 ChST 2023
;; MSG SIZE  rcvd: 452
````
MX records are mail exchange, but doesn't list the pop3 server??
Be interesting if you check domain and find they do have a pop3 server running.