A maximum time execution error message indicates that your Gmail draft size is too big. Here are below some recommendations.
Why do I get this error?
That happens because your email draft is too big, usually because of big inline images or attachments.
We recommend uploading 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 Photos, 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 list box displaying all your drafts:
The same goes for attachments. If you want to use attachments larger than 1MB, we recommend the upload them on Google Drive and 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 delete them, so the issue remains in the existing draft).