Gmail actually makes it simple to send outbound mail from your MailRoute SMTP Auth account!  You simply configure another outbound email address and associated server, and the gmail webclient uses it whenever you're sending email from that account.

TL;DR (Too Long, Didn't Read)

Gather the info from our Control Panel by clicking the SMTP Auth Relay tab on the left menu.  You'll have to click the eye icon to view your passwords.  If you don't have a local password set up, you'll be asked to create one.  The Inbound (IMAP) and Outbound (SMTP) servers may have different logins and will have different passwords.   Go into your Gmail Settings -> See All Settings -> Accounts and Import, and add an address in the Send mail as section.

When you send email, click the From address to choose your new account for sending email.  It'll pass SPF, DKIM, DMARC checks.  Congratulations!

Long Version

Gather the info you need to configure your email client

To configure your email client to use the MailRoute SMTP Auth Relay service for outbound email relay, you will need to gather some info from the MailRoute Control Panel and then configure your email client. 

1. Login to admin.mailroute.net and click the My Settings link at the top of the page.
   
   
2. Click the SMTP Auth Relay tab in the Settings menu on the left side of the page. 
   
   
   
   
3. Gather your info!  You can click the copy icon  to copy the values of any field.
   
   For the Gmail webmail interface, you will only need the info in the Outbound (SMTP) Server configuration.
   
   
   Outbound (SMTP) Server Password:  Click the eye icon to display your password. Accept the Terms of Service:
   
            
   
   By the way, you can generate a new password for your Outbound (SMTP) Server at any time.

Configure your email client

1. Go into Gmail on a web browser.  Choose the Setting icon at the top, and then click on See all settings:
   
   
   
   
2. Choose Accounts and Import, and then click Add another email address... from the Send mail as section
   
   
   
3. Enter your Full name and your new Outbound SMTP Auth email address.
   
   If you use the Gmail webmail client as the primary client for your email address, you may want to select Treat as an alias. If, instead, you just treat your account as a way to send email on that account's behalf, then do not select Treat as an alias
   
   
   NOTE: Treat as Alias doesn't affect the reception of the message, nor the
   sending of the message
   
   It does affect how gmail views the message, and how it categorizes it. Its
   your choice which way you prefer.
   
   Treat as alias applies to addresses you use for sending mail, but addresses
   you use for sending email fall into at least two classes, "my addresses"
   and "other addresses I am allowed to use" (ie, if you are sending email
   on another account's behalf). "Treat as alias" is used to identify
   "my addresses".   
   
   The primary use case for "treat as alias" is for external addresses you
   have delivered to your Gmail account, so you are using Gmail as the
   primary client for the address.
   
   So it all depends on how you want to use gmail for this account.
   It's up to you.
   
   To summarize:
   - External addresses that are not aliases are treated as someone else's address.
   - If you send a message to an external address that is not an alias,
   it will not show in the Inbox.
   - If you receive a message from an external address that is not an alias,
   it will not show in Sent.
   - When you reply to a message from an external address that is not an
   alias, it is a normal reply and will be addressed to the original sender
   - When you search for from:me or to:me, it will not match external
   addresses that are not aliases.
   
   Click Next Step>
   
4. Here's where you start filling in the info you gathered from our Control Panel.  Enter the following:
   
        SMTP Server: <Your Outbound (SMTP) Server>
        Port: <Your Outbound (SMTP) Port>
        Username: <Your Outbound (SMTP) Login>
        Password: <Your Outbound (SMTP) Login>
   
   Make sure Secured connection using TLS (recommended) is selected.
   
   Then click Add Account >
   
5. You'll see a verification window pop up.  Google will send an email to your address with a confirmation link.  Go into your email client, find that email and click on the confirmation link.
   
6. Your Gmail settings will now show a verified new address.  You may want to choose the option to Reply from the same address the message was sent to:
   

Using your new account to send email

When you compose a new email, you can choose to send it from your new account.  Just click on the address in the From header and choose the account from which you would like to send the message.



That's all there is to it!  Have fun!