Client can’t connect to mail server with SMTP (postfix)

I have a mail server with postfix and dovecot. The mail server is on a VPS. At the moment, I can send/receive mails from the server itself. It seems to work OK.

However, I'd like to use my mail addresses from my laptop. I configured my account, set up the IMAP and SMTP. IMAP works, I can receive my mails on my laptop.

But I can't send any mail.

I looked up the traffic using tcpdump, I could see SMTP packets going out of my laptop, but nothing comes at my VPS.

After a few research, it seems that my ISP is blocking that traffic. What am I supposed to do? It seems strange, because we can use SMTP to send mails with gmail and other mail services.

I used the online tool mxtoolbox and everything seems ok

+------+---------------------------+------------------------------------------+
|      | Test                      | Result                                   |
+------+---------------------------+------------------------------------------+
| Warn | SMTP Reverse DNS Mismatch | Reverse DNS does not contain the hostname|
| Warn | SMTP Banner Check         | Reverse DNS does not match SMTP Banner   |
|  OK  | SMTP Valid Hostname       | OK - Reverse DNS is a valid Hostname     |
|  OK  | SMTP TLS                  | OK - Supports TLS.                       |
|  OK  | SMTP Connection Time      | 1.000 seconds - Good on Connection time  |
|  OK  | SMTP Open Relay           | OK - Not an open relay.                  |
|  OK  | SMTP Transaction Time     | 3.047 seconds - Good on Transaction Time |
+------+---------------------------+------------------------------------------+

Connecting to W.X.Y.Z

220 example.com ESMTP Postfix [641 ms]
EHLO PWS3.mxtoolbox.com
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN [656 ms]
MAIL FROM:<supertool@mxtoolbox.com>
250 2.1.0 Ok [656 ms]
RCPT TO:<test@example.com>
451 4.3.0 <test@example.com>: Temporary lookup failure [656 ms]

PWS3v2 7391ms