There are 2 main methods for sending/testing outgoing mail:
  * __Telnet/nc__
  * __swaks__
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My favourite is using swaks as its easy to use and quick. You will probably need to install though:
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==== SWAKS ====
Swaks can be found in the epel repository. Make sure you have this repo enabled and then run:
<sxh bash>
# yum install swaks -y
</sxh>
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=== Testing your local mail server ===
The following command tests your local mail configuration (postfix). This includes the relay that may be configured with it. 
<sxh bash>
swaks --to random_email@gmail.com --server 127.0.0.1 -from luke@example.com
</sxh>
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=== Testing the relay host ===
The following command will send mail directly to the relay (for example sendgrid or mailgun). 
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Change "username" to the username you normally authenticate with to the relay host and you will be prompted for a password once you run the following command:
<sxh bash>
swaks --to random_email@gmail.com --server smtp.sendgrid.net:587 --auth-user username -tls
</sxh>
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==== Telnet ====
Command:
<sxh bash>
telnet localhost 25
</sxh>
Response:
<sxh bash>
Trying ::1...
Connected to localhost.
Escape character is '^]'.
220 cloud-server-03.localdomain ESMTP Postfix
</sxh>
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Command
<sxh bash>
HELO localhost
</sxh>
Response:
<sxh bash>
250 cloud-server-03.localdomain
</sxh>
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Command
<sxh bash>
mail from: luke@localdomain.com
</sxh>
Response:
<sxh bash>
250 2.1.0 Ok
</sxh>
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Command
<sxh bash>
rcpt to: luke@lukeslinux.co.uk
</sxh>
Response:
<sxh bash>
250 2.1.5 Ok
data
354 End data with <CR><LF>.<CR><LF>
</sxh>
Type some data and then end the email with a `.` on a line by itself. 
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Press enter to escape and the type quit