I finally got around to reading my PostFix logs, and I found a few big errors. One of them was fatal: address(##): message file too big
The problem is probably in these two settings in the PostFix main config: message_size_limit and mailbox_size_limit
Check the configuration using:
postconf | grep size postconf -d | grep size
where the -d is to check the default size.
Fix the error by editing the file using:
Method 1:
sudo postconf -e mailbox_size_limit=0 sudo postconf -e message_size_limit=0
Method 2:
sudo nano /etc/postfix/main.cf
Either add these lines or edit existing ones
message_size_limit = 0 mailbox_size_limit = 0
Restart
service postfix restart