That’s interesting as usually mailing is working and gmail needs special care like working SPF, DKIM etc…
Just to exclude missing IPv4/IPv6 issues, could you please check /etc/hosts if the localhost entries (127.0.0.1, ::1) are correct, see also One NethServer environment cannot send email - #6 by mrmarkuz
Could you please share the mail app logs when sending/receving is not working to get the error?
Did you set right identities for sending with other accounts than gmail? For example you can’t send via gmail using the mail address “user@outlook.com”.