A maximum time execution error messages indicates that your Gmail draft size is too big. Here are below some recommendations.
Why do I get this error?
This happens because your email draft is too big. Usually because of big inline images or attachments.
We recommend to upload in the email only images that weight less than 500kb. If you want to add bigger images, best is to upload them elsewhere (on Google+ Photo, for example) and then insert the image into your email.
Before sending an email with YAMM, you can check the size of your draft directly from the listbox displaying all your drafts:
The same goes for attachments. If you want to use attachments bigger than 1mb, we recommend the upload them on Google Drive and simply add a link to the file in the email.
Note that the best is to create a new draft in Gmail rather than deleting all big images in your current draft (Gmail tends to hide them rather than actually delete them so the issue remains in the existing draft).