If you are getting following or similar exception:
“A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond” or “No connection could be made because the target machine actively refused it.” most likely you provided incorrect server, port, and SSL usage configuration to Connect, or ConnectSSL methods.

Most email providers (like Gmail, Hotmail, Yahoo and others) use default ports. Connect() and ConnectSSL() and fluent interface use those ports when not specified otherwise.

Default ports for email protocols are:

Please ask your email server administrator for server, port, and SSL usage,
if those are not standard there is no way you can guess them.

Please also make sure that:

• you are using correct client class with protocol and port you plan to use (Pop3 class with POP3 protocol, Imap class with IMAP, and Smtp class with SMTP protocol)
• you are using ConnectSSL when you require SSL, and Connect when you don’t
• firewall and antivirus software are disabled or configured correctly
• the protocol you are trying to use is enabled on the server. Exchange or Gmail can have some protocols disabled by default

Following articles can help you with that:

• How to enable POP3 in Gmail
• How to enable IMAP in Gmail
• How to enable IMAP for Exchange

Establishing connection using default port is simple:

// C#

client.Connect("mail.example.com");

' VB.NET

client.Connect("mail.example.com")

or if SSL is enabled:

// C#

client.ConnectSSL("mail.example.com");

' VB.NET

client.ConnectSSL("mail.example.com")

If you need to specify different port just use overloaded version of Connect method:

// C#

client.Connect("mail.example.com", 999);

' VB.NET

client.Connect("mail.example.com", 999)

If you are using SSL:

// C#

client.ConnectSSL("mail.example.com", 999);

' VB.NET

client.ConnectSSL("mail.example.com", 999)

If you need to specify different port using fluent interface use OnPort() method:

// C# version

Mail.Text(@"Hello")
  .To("to@mail.com")
  .From("from@mail.com")
  .Subject("Subject")
  .UsingNewSmtp()
  .Server(_server)
  .WithSSL()
  .OnPort(443)
  .WithCredentials("user", "password")
  .Send();

Some servers require explicit SSL (Usually this triggers “The handshake failed due to an unexpected packet format” exception) please look at following articles for details:

• Use SSL with IMAP
• Use SSL with POP3
• Use SSL with SMTP

If you have problems with SSL, your server may be using self-signed certificates:

• The remote certificate is invalid according to the validation procedure

Again when in doubt please ask your email server administrator for server, port, and SSL usage.