You can use your mail app on your iPhone
to block unwanted emails from certain email addresses.
This even works with an external connected email account,
for example, a Gmail account.
Open your mail app on your iPhone.
Navigate to an email
from the email address you want to block.
Press on the sender's name.
Press the name again
to open up the additional options menu.
Press "Block this Contact."
Confirm the block by pressing "Block this Contact."
Click "Done."
Now you'll see a red circle with a line through it
in the header box and a message saying,
"This message is from a sender in your blocked list."
If the sender sends you another message
from this point further,
it will automatically be placed into the trash
of the email account it's being sent to.