Check out this forum thread I came across some time ago.
They used a local postfix to relay SMTP. In theory you can use multiple SMTPs with this trick - FOR FREE
thanks for the link. but this is not an option unless you run your own smtp server yourself.
anyways - I dont mean to go back and fourth. I think I see the writing on the wall for the issue. just adding a +1 and moving on.
I noticed, there is an update for M5.
If you need to have multiple SMTP from the same Mautic, for example for cold emails and business emails, this is the best solution.
I have created a simple multiple SMTP plugin that is only compatible with file queues. You can try. GitHub - HN-PRO/MauticEvenlyDistributesSmtpBundle: Evenly Distributes SMTP Server For Mautic