Nrthugu

We have a site on Google Cloud, let's call it 'main.co.uk'

and a subdomain site called 'forum'.

We are using Gmail business for main.co.uk emails, verified by MX records, SPF etc

Now what's confusing me is using mailgun for sending mail for 'forum'.

I know we can have multiple MX Records for main.co.uk but is there a good way to do this? do we set different priorities? Will we get issues with mailgun and Google trying to handle incoming mail?

Any advice would be great.

Using mailgun for sending emails does not involve MX records. Services like mailgun will use credentials for one of your email servers and will act like an email client program. Mailgun will authenticate and then upload email to the email server. There is also the option for programs like mailgun to act as an SMTP server for your domain (you can have more than one sending server).

Mailgun can also be configured as an email server for receiving email. However, you do not replace your existing email server, instead you create a subdomain that is then managed by mailgun. In this use case all incoming emails for that subdomain someone@mailgun.example.com are then processed by mailgun. This is similar to having multiple email accounts that you need to login into to read your email. The intent here is for mailgun to apply intelligence to the sending and processing of your email campaigns by managing email bounces, click throughs, etc.

MX records specify the mail server responsible for accepting email. You can have multiple MX records with different priorities but they are pointing to the same email system (collection of servers storing your inbox), not to different servers at different providers. For example, you would not have one MX record point to Gmail and another MX record pointing to Office 365 (or mailgun and Google). Multiple MX records support fault tolerance and failover, not multiple providers.

You can have an email server setup for main.co.uk and another email server setup for forum.main.co.uk but these are separate email server setups (I am ignoring email aliasing). You can have mailgun send email for someone@forum.main.co.uk with a return address anotherperson@main.co.uk. Normally you want to keep the sender address and return address the same so that SPAM filters don't kick in.

In summary, use mailgun to send emails from your website and / or email marketing campaigns and a normal email system (Office 365, Gmail, etc.) for everything else but have them setup as separate independent email systems.