If you are using an email client (e.g. Microsoft Outlook, Mozilla Thunderbird, Mac email client, etc.) and you can’t send emails while connected to our VPN server, please try to close your email client and start it again. This will solve the problem most of the times. If the issue persists, check your email client outgoing and incoming settings.
Please reconfigure your mail’s outgoing and incoming settings. You should not use port 25 to send your mail (the default SMTP port used by servers to communicate with each other). Please use port 587 and STARTTLS to send/relay email through VPN. It helps to prevent outgoing spam.
Explanation: Port 25 is used by MTA to MTA communication (mail server to mail server). It could be used for client to server communication but it is not currently the most recommended. Standard SMTP port 25 accepts email from other mail servers to its “internal” mailboxes without authentication, and  therefore it is susceptible to SPAM.
Make sure to always use encrypted traffic, to protect your privacy, data integrity and also for security reasons. You can find the list of default ports used for sending and receiving email below,that are both encrypted and unencrypted:
IMAP uses port 143, SSL/TLS encrypted IMAP uses port 993.
POP uses port 110, SSL/TLS encrypted POP uses port 995.
SMTP uses port 25, SSL/TLS encrypted SMTP uses port 465 or 587.
More detailed information can be found in this article