I am using MailGun for sending emails and Zoho for receiving emails, which works fine.
But then I wanted to enable sending from Zoho to be able to reply to received emails, so I added Zoho spf and DKIM to the domain along with MailGun, which also worked perfect emails from both servers are signed and sent correctly.
My question: is there any drawback from doing this? like for example domain get blacklisted, or flaged as spam?
My Second Question: why people dont do this? I can even add a third smtp server and separate email tasks: 1st server for transactional emails, 2nd server for bulk, 3rd server for receiving.
First question:
As long as both SMTP servers are added to your SPF records, and use DKIM, there is absolutely no reason why your emails will be marked as spam (unless the content of the emails are spammy and are picked up through content filters). Email security protocol isn't very rigorous and SPF and DKIM are responsible for a large amount of detected spam.
Second question:
I'm sure a lot of people do actually employ this-- a quick google search backs this up. Most people aren't using SMTP for bulk email. Maximizing the output of an SMTP server usually doesn't require more than one, but it certainly doesn't mean it won't be practical.