Flask mail emails delayed on not sent to some domains

I am building a Flask app, and while developing, I configured Flask-Mail to send emails through a Gmail account on port 25. With that configuration, I am able to send emails to other Gmail accounts without a problem. However, when I switch the configuration to a email address hosted on my offices mail server over port 25, emails sent to Gmail accounts are drastically delayed (+10min) or are never received. Thanks in advance for any insight!