Most likely Mail is going to SPAM, Mail is defiantly getting sent, In your logs you see  250: 2.0.0 Ok: queued as which means the Mail Server you are sending Mail too has accepted the Mail.

Do you have DMARC, DKIM, SPF implemented, have you implemented PTR Record that matches the hostname & IP of the Mail Server?

Regards

So still can't sent mail
Aug 23 19:10:41 mail roundcube: <4l7lsrfp> User mbetton@fast.com [107.77.194.145]; Message <dfffa15d5eed4def1465f66a29bafab6@fast.com> for fast@gmail.com; 250: 2.0.0 Ok: queued as 4WrHQT3Rtvz42XK
Aug 23 19:10:41 mail postfix/submission/smtpd[5105]: disconnect from mail.fast.com[127.0.0.1] ehlo=2 starttls=1 auth=1 mail=1 rcpt=1 data=1 quit=1 commands=8
Aug 23 19:11:03 mail postfix/10025/smtpd[5155]: connect from mail.fast.com[127.0.0.1]
Aug 23 19:11:03 mail postfix/10025/smtpd[5155]: discarding EHLO keywords: CHUNKING
Aug 23 19:11:03 mail postfix/10025/smtpd[5155]: 4WrHQv3fglz43H5: client=mail.fast.com[127.0.0.1]
Aug 23 19:11:03 mail postfix/cleanup[5115]: 4WrHQv3fglz43H5: message-id=<dfffa15d5eed4def1465f66a29bafab6@fast.com>
Aug 23 19:11:03 mail postfix/10025/smtpd[5155]: disconnect from mail.fast.com[127.0.0.1] ehlo=1 mail=1 rcpt=1 data=1 quit=1 commands=5
Aug 23 19:11:03 mail postfix/qmgr[2570]: 4WrHQv3fglz43H5: from=<mbetton@fast.com>, size=5196, nrcpt=1 (queue active)
Aug 23 19:11:03 mail amavis[2579]: (02579-01) Passed CLEAN {RelayedInternal}, ORIGINATING/MYNETS LOCAL [127.0.0.1]:51586 ESMTP/ESMTP <mbetton@fast.com> -> <fast@gmail.com>, (), Queue-ID: 4WrHQT3Rtvz42XK, Message-ID: <dfffa15d5eed4def1465f66a29bafab6@fast.com>, mail_id: kRuyDCAtwC0p, b: hiJYq9N7x, Hits: -1.01, size: 4024, queued_as: 4WrHQv3fglz43H5, Subject: "Fwd: ⏰ Hurry - Sale Ends Tonight! (raw: =?UTF-8?Q?Fwd=3A_=E2=8F=B0_Hurry_-_Sale_Ends_Tonight!?=)", From: <mbetton@fast.com>, User-Agent: Roundcube_Webmail, helo=localhost, Tests: [ALL_TRUSTED=-1,T_SCC_BODY_TEXT_LINE=-0.01], autolearn=ham autolearn_force=no, autolearnscore=-0.01, dkim_new=dkim:fast.com, 21799 ms
Aug 23 19:11:03 mail postfix/amavis/smtp[5120]: 4WrHQT3Rtvz42XK: to=<fast@gmail.com>, relay=127.0.0.1[127.0.0.1]:10026, delay=22, delays=0.23/0.2/0.03/22, dsn=2.0.0, status=sent (250 2.0.0 from MTA(smtp:[127.0.0.1]:10025): 250 2.0.0 Ok: queued as 4WrHQv3fglz43H5)
Aug 23 19:11:03 mail postfix/qmgr[2570]: 4WrHQT3Rtvz42XK: removed
Aug 23 19:11:48 mail postfix/anvil[4975]: statistics: max connection rate 1/60s for (smtpd:60.211.206.17) at Aug 23 19:08:27
Aug 23 19:11:48 mail postfix/anvil[4975]: statistics: max connection count 1 for (smtpd:60.211.206.17) at Aug 23 19:08:27
Aug 23 19:11:48 mail postfix/anvil[4975]: statistics: max cache size 1 at Aug 23 19:08:27
Aug 23 19:13:04 mail postfix/smtp[5156]: connect to gmail-smtp-in.l.google.com[142.250.114.27]:25: Connection timed out
Aug 23 19:13:34 mail postfix/smtp[5156]: connect to alt1.gmail-smtp-in.l.google.com[142.250.152.27]:25: Connection timed out
Aug 23 19:14:04 mail postfix/smtp[5156]: connect to alt2.gmail-smtp-in.l.google.com[172.253.113.26]:25: Connection timed out
Aug 23 19:14:05 mail postfix/smtp[5156]: 4WrHQv3fglz43H5: to=<fast@gmail.com>, relay=none, delay=181, delays=0.02/0.19/181/0, dsn=4.4.1, status=deferred (connect to alt2.gmail-smtp-in.l.google.com[172.253.113.26]:25: Connection timed out)

telnet smtp.gmail.com 25
telnet: could not resolve smtp.gmail.com/25: Temporary failure in name resolution

FROM DESKTOP
dig mx gmail.com

; <<>> DiG 9.18.28-0ubuntu0.22.04.1-Ubuntu <<>> mx gmail.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 24451
;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;gmail.com.            IN    MX

;; ANSWER SECTION:
gmail.com.        2672    IN    MX    10 alt1.gmail-smtp-in.l.google.com.
gmail.com.        2672    IN    MX    5 gmail-smtp-in.l.google.com.
gmail.com.        2672    IN    MX    40 alt4.gmail-smtp-in.l.google.com.
gmail.com.        2672    IN    MX    30 alt3.gmail-smtp-in.l.google.com.
gmail.com.        2672    IN    MX    20 alt2.gmail-smtp-in.l.google.com.

;; Query time: 96 msec
;; SERVER: 127.0.0.53#53(127.0.0.53) (UDP)
;; WHEN: Fri Aug 23 19:51:35 CDT 2024
;; MSG SIZE  rcvd: 161

host -t mx gmail.com
gmail.com mail is handled by 40 alt4.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 5 gmail-smtp-in.l.google.com.
gmail.com mail is handled by 20 alt2.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 10 alt1.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 30 alt3.gmail-smtp-in.l.google.com.

FROM MAIL SERVER
root@mail:/home/fast# dig mx gmail.com
;; communications error to 8.8.8.8#53: timed out
;; communications error to 8.8.8.8#53: timed out
;; communications error to 8.8.8.8#53: timed out
;; communications error to 8.8.4.4#53: timed out
;; communications error to 127.0.0.53#53: timed out

; <<>> DiG 9.18.28-0ubuntu0.22.04.1-Ubuntu <<>> mx gmail.com
;; global options: +cmd
;; no servers could be reached

root@mail:/home/fast# host -t mx gmail.com
;; communications error to 8.8.8.8#53: timed out
;; communications error to 8.8.8.8#53: timed out
;; communications error to 8.8.4.4#53: timed out
gmail.com mail is handled by 20 alt2.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 10 alt1.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 5 gmail-smtp-in.l.google.com.
gmail.com mail is handled by 30 alt3.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 40 alt4.gmail-smtp-in.l.google.com.

ActiveCampaign TO TEST ONLY
SPF/DKIM/DMARC Results
Domain    DKIM       SPF         DMARC       
fast    fail      OK    OK    Retest   
We could not find any DKIM records.

Gmail and Yahoo require DKIM for all sending domains if you send more than 5,000/day. Therefore, ActiveCampaign strongly recommends all customers set up DKIM for all sending domains.

More info here

You have an SPF record but it does not whitelist our IPs. That is not a problem because ActiveCampaign manages SPF for you via your Mailserver Domain.

More info here

This is your current SPF record:
v=spf1 ip4:68.x.x.x/29 a:mail.fast -all
Your DMARC record looks great!

Your actual DMARC record is:
v=DMARC1;p=none;rua=mailto:a1aacd2199@rua.easydmarc.us,mailto:fast@fast;ruf=mailto:a1aacd2199@ruf.easydmarc.us,mailto:fast@fast;fo=1;

DKIM
    Test    Result
Status Ok    DKIM Record Published    DKIM Record found
Status Ok    DKIM Syntax Check    The record is valid
Status Ok    DKIM Public Key Check    Public key is present

Please HELP