Q: The email server didn’t recognize your username/password combination.

Use iCloud Photo Sharing:

iCloud Photo Sharing FAQ - Apple Support

Mail

Dropbox

Flickr

SharedPhotoStreams

Facebook

Many ways - most likely the issue is with two step log in in Gmail - to use Gmail with two step login you have to set iphoto as an exception

LN

If two-step verification is the problem as LarryHN suspects, you need to set up an app-specific password for iPhoto.

Other forum members reported, that creating  an app-specific password is required, see the post below:

Re: I am using iPhoto version 9.6 and I can no longer get an email sent with a photo attached. I get an error message saying the email did not go through because the server did not reply. I can send the photo with Mail. Help!in response to Gary Kissler| This solved my question

This solution is for those of you using 2-step verification for your Apple ID and are using iCloud as the mail server that you are trying to mail photos with.

 

I too had this issue. The problem in my case occurred because I had setup 2-step authentication for my Apple ID. If you have done this then you will get the error message "the mail server did not recognize your email/password...". To solve the problem go to:

 

https://appleid.apple.com

 

• Click "Manage Your Apple ID"
• Verify your identity with the device you selected (if this step does not show up then you have not likely set up 2-step authentication and this is not your solution)
• Click Password and Security in the left column
• Click Generate an app-specific password
• Enter iPhoto as the name of the app
• Copy the password and paste this into the password box in iPhoto where you would normally enter your apple ID (in the mail account section in iPhoto preferences).

 

And...voila' (I hope)

in the iPhoto preferences ==> accounts delete your e-mail account and reenter it

OR

IMHO the better solution is to set Apple mail as the email client in the iPhoto preferences

LN