Our company has been struggling to migrate Exchange to GMail for last 2 years (I will leave this for /r/talesfromtechsupport). We have two domains on two different ccTLDs, and lately Exchange is just forwarding messages to GMail.
I think it is the right time to change MX records, so that GMail effectively would host both domains. However, as people are still using Exchange for Public Folder storage, once in a while they will send messages from the old domain.
I was wondering whether it can impact the message delivery? I think it has to do something with SPF, but resources are slightly dry and my competence is low.
Perhaps there is a way to disable SMTP on Exchange altogether, but in a way that users wouldn't have the option?
Thank you all for help.
SPF typically only affects outbound message delivery, so based on a concern purely of incoming mail delivery and SPF then there's nothing to worry about.
And providing users are using Gmail to send messages, and your SPF records are in place (or are absent) and there are no problems, then again there's nothing to worry about.
Try using a tool like Wormly SMTP test tool to pipe a message in manually into Gmail's SMTP server in the way you think messages will be delivered, and if that works then really there's no reason that I can think of why you shouldn't change your MX records.
Keep your existing Exchange servers in there with a lower priority (e.g. priority 20, with GMail having priority 10) just in case there's something unforseen...