If your emails are getting blocked by your recipients, or constantly being marked as Spam mail, you may refer to the following dos and don'ts when sending email.
Sending too many emails a day/hour/minute may cause your email server to be blocked by anti-spam services.
Purchased mailing list usually do not come with recipients' consent, and they may complain or block your email if they receive one.
If you are sending marketing emails, make sure your recipients have provided their consent to receive your emails, or they have signed up for your mailings willingly.
It is important to remove non-existent or opt-out email addresses from your mailing list so that you won't keep sending to invalid email addresses, which could trigger spam block.
Always include a functional unsubscribe link in your emails, to avoid emails being flagged as spam when received by users who no longer want to get your emails.
Check your email content and minimize grammatical mistakes and spelling errors before sending out. Emails with poor spelling and grammatical errors will usually alert spam filters.
One of the important guidelines of sending commercial emails (CAN-SPAM Act) is never use the words “no reply”, or similar phrase, as your sender email address.
Sending emails to few hundreds of contacts at once will trigger spam filters to block your mail server instantly. Such block may be temporary (few hours to few days), or permanent.