If there is anything to rely on when writing and reading your own texts, it is authorial blindness. Have the e-mail read by other, unbiased eyes before sending it. If you don't have that option, you can copy the text, for example, into MS Word and turn on automatic spell checking (just don't forget to set Czech as the language). It won't reveal everything and you can't rely on it 100%, but it will definitely help you catch the biggest mistakes. We probably don't need to emphasize that errors and typos in texts (and not only in newsletters) reduce your professionalism in the eyes of a potential customer.
Tip: Some people may find it helpful to reformat the text when checking for typos and errors. By enlarging the font or changing the font, you can fool yourself for a moment and you may see flaws in the text that netherlands email list you missed on the first (or even fifth) read.
Missing image alt text
Newsletters containing images are more popular with readers than purely text-based emails. However, it is necessary to take into account the possibility that the images will not be downloaded due to a technical error and will not be displayed at all. A correctly filled-in alt description can be a lifesaver , which will then appear to recipients instead of the image. Do not forget about it, especially if your image contains text. Thanks to the alt description, the newsletter will make sense as a whole even in such cases and will not confuse your potential customers.
Unresponsive newsletter
Do you also open most of your newsletters on mobile? Your email readers probably do the same. Think about it and test how your newsletter will look on which device. If you use emailing tools like SmartEmailing, Ecomail or any other, you can already see how it will look on mobile, tablet and desktop while creating your newsletter.