In game mail blocked.
It is exactly what the message you get says. There is a limit to how many mails you can send out in a fixed period of time. Many think it is too tight of a throttle, many don’t even know it is there (like you) but the company has not changed it in years.
You can not send excessive messages in a short amount of time. Just one of the limitations associated with the in-game mail feature.
Okay, I didn’t know that. So what’s the limit over what amount of time?
If you send more than 2 mails within a couple of minutes, you’ll get supressed. Every time you try to send a mail, the cooldown timer gets reset (I think!).
So, send 2 mails, wait a couple of minutes. Rinse and repeat.
If you have a guild bank, you can more easily bulk exchange between players there. Of course, if the bank is shared with other guildies (i.e. isn’t a personal/family guild) you need to make sure that they’re trustworthy/sensible/know that you’re exchanging just between the two of you for that time period.