E-mail sent to the invalid e-mail address
Sending an e-mail to an e-mail address that is misspelled or does not exist results in that e-mail not being delivered. While it seems obvious, it is easily overlooked and happens often.
Even if it's spelled correctly if the e-mail address does not exist, the e-mail cannot be delivered and you will usually receive a non-delivery notification. The notification will tell you the e-mail address does not exist, isn't found, or other important information.
To resolve these issues, verify the e-mail address is spelled correctly and that it is valid. You may want to contact the recipient by phone to confirm the spelling and validity of the e-mail address.
E-mail sent to Junk folder
E-mails can sometimes be sent to a Junk folder in the recipient's account without them realizing it. The cause could be the recipient previously flagged your e-mails as junk by accident, resulting in all future e-mails being diverted to the Junk folder.
The e-mail client or e-mail service may also flag the e-mail as graymail, a type of e-mail not considered as spam, but also not considered an important e-mail either. E-mails flagged as graymail often are sent to the Junk folder.
To resolve an issue with e-mail getting sent to a Junk folder, ask the recipient to check their Junk folder. If the e-mail is found there, they can flag it as "not Junk," which allows messages once again to go to the recipient's Inbox.
E-mail marked as spam by e-mail provider
Many e-mail providers have anti-spam protection built into their e-mail service. The anti-spam protection attempts to detect if an e-mail is a spam, and if so, block it from being delivered. Sometimes the anti-spam protection feature gets it wrong, however, and incorrectly marks messages as spam.
If the e-mail is getting sent to a business, it may have an e-mail security system that works to detect spam e-mails. Like the anti-spam protection of an e-mail provider, a security system will try to determine if an e-mail is a spam and then block it accordingly.
To resolve an incorrect spam detection, ask the recipient to request that their e-mail provider or security system whitelist your e-mail address.
Hope You Find This Useful,
Peter