Update attachment message on mailing form.
The Attachment tab on the Mailing page displays a message on the screen to add new files. But there is no indication of the size of the file which could be attached here. Larger file attachments lead to failure in the mailing process and eventually results in not sending mails to the recipients.
The above message should be displayed something like -
Alternatively, you may add new files using drag/drop. Each file must be less than <max_size>M in size. For larger files, please upload it somewhere else and paste a link in the body of this mailing.
Normal email form do have this specified -