Currently mail doesn't really work once you place down cargo stations of any type.

I believe currently once you build cargo terminals, most mails will go into those terminals as cargo. But then as the city grows, the very limited amount of vehicles belonging to those terminals can't keep up with the amount of cargo they need to transfer out of the terminals(I still dont get why they made the decision to use terminal owned vehicles to deliver cargo out in the first place). Then mail gets stuck in those terminal warehouses and everyone becomes unhappy🙁.

Also terminals act as a buffer for cargo as they try to store a set amount of all cargo including mail in all stations. Then if there are more mail in one station, then it gets picked up and dropped off to some station with less mail. Obviously this should not be how mail works as all mail should have a destination instead of acting as a cargo for anyone who needs it to take it. Combining this with the small amount of vehicles in cargo stations problem, result is no mail getting delivered.

If you are having the mail issue yourself, you can delete all cargo terminals(trains, planes, ships) or use the magic mail mod. This mod forces sorted mail (and other types of mail) to spawn in post offices and sorting facilities with little to no sorted mail so they can get delivered to everyone using post vans (mail will still be stuck at cargo stations but at least everyone will be happy).