SMTP error 535 means your login credentials are incorrect, so please ensure you have entered the correct details into the app settings. The settings you must verify are in Settings > General > Photo Settings > Email, and then in the Login section please enter you full Gmail address and case sensitive password.
SMTP error 534 is supposed to mean the message is too big, however I have seen this fixed in two ways not relating to file size. First is to double check your login credentials as described above for SMTP error 535. The other is that you may have 2 factor authentication enabled on your Google Account and the protocol within the app used to send the email is not capable of handling that, hence you may need to disable it on your Google Account if you wish to use this functionality. You may find that is what also causes SMTP error 535 in some cases.
Essentially, if you receive either of these SMTP error messages, please double check you Gmail login credentials and ensure you are not using 2 factor authentication. If you receive any other SMTP errors, simply perform a Google search ("SMTP error nnn") for that particular error code.
