You are sending files to big.
No matter what or wich mail server you use, mails won't arrived, do of the limit of data allocated to the recipient account.
Tray to send 3 or 4 pictures at the time on severals emails(total around 1Mb each).
If the recipient account has a limit to low, lets say 2Mb, the second or third email, never will get there, unless the client empty the inbox.
Ask the client to open a gmail account or look on the possibility of FTP.
Set PM to Review email application before sending (you can see, the total data you are sending)
Set Outlook to plain text
Hope this help