Most SMTP (Outgoing) email servers now require you to have specific features enabled with your smtp account login, so that the receiving email server can trust the content in the email and that it is not likely to be spam.  If it is not configured correctly you may be getting the following error when you send an email:

The best option is to ensure that your domain name has the correct DMARC, DKIM & SPF settings (talk to your IT provider to check these are enabled).

If you are using the SMTP2GO outgoing email service, you should also use the DMARC/DKIM/SPF settings, but you can also just verify the FROM: email address that you are using to send your emails with.  Note that the domain verification is preferential as the single email verification will include the words "via smtpcorp.com" in the header area of the email.

To check whether the SMTP2GO account is verified:

1. Login to your SMTP2GO account
2. Go to Sending > Verified Senders
3. The first view is of your 'Sender Domain' status.  If the domain is verified it will appear with a green background, otherwise it will be orange
   
   
   Use the 'Add Sender Domain' button for the details that your IT provider will need to add.
   
   
4. You can also click on the 'Single sender emails' link
   
   
5. Use the 'Add single sender email' button and enter the email address that you are sender your emails from.  Then check that you have received the email and click on the Verified option in the email.
   
   
   It will then appear in the SMTP2GO screen as being verified