Long story short: You can't.
Choosing an email sender as the "from" address is really important when it comes to deliverability. To give your email the best possible chance to reach the inbox and bypass the spam folder, you need to avoid free webmail address as your sender address like Gmail, Yahoo, Outlook/Hotmail or AOL. Instead, use an email address registered with your organization's own domain.
Why did we decide to block the option?
For 5 simple reasons:
- Increase your deliverability and prevent your emails to end in the spam folder.
- Encourage the best practices of email marketing.
- Make you look more professional.
- Prevent misleading addresses and block spammers.
- Most importantly, you’ll have a lot more control over your email address’ reputation.
Why free webmail like Gmail goes to spam?
When using a third party email service provider (ESP) like MoonMail, all your emails go with a signature "mailed by", in our case all emails are mailed by "amazonses.com".
If you send an email using a free webmail "From" addresses like Gmail, the email is sent with the signature and it's considered as more suspicious than those from custom domains and it's sent to the spam folder. This is the case no matter which ESP you use.
You can set up the domain verification, SPF and DKIM records to tell Gmail that you authorize MoonMail to send emails on your behalf. In free webmail accounts you CAN'T set up those verification records, so you can never give the trust for your email to hit the inbox.
Btw, there's more sender domains which aren't allowed to be used in MoonMail but are not disclosed for security purposes.
Related article: How to get TXT, SPF and DKIM records from your MoonMail account