plot = ListPlot[{{0.25, 0.25}, {0.5, 0.5}, {0.75, 0.75}},
   Frame -> True, PlotRange -> {{0, 1}, {0, 1}}, Joined -> True,
   AspectRatio -> 1,
   FrameLabel -> {{"y (mm)", ""}, {"x (mm)",
      "aaaaaaaaaabbbbbbbbbbccc"}},
   BaseStyle -> {FontWeight -> "Bold", FontSize -> 40,
     FontFamily -> "Calibri"}, ImageSize -> 800];

Export["F:\export23.png", plot, "png"];

ImportString[
 ExportString[
  ListPlot[{{0.25, 0.25}, {0.5, 0.5}, {0.75, 0.75}}, Frame -> True,
   PlotRange -> {{0, 1}, {0, 1}},
   Joined -> True, AspectRatio -> 1,
   FrameLabel -> {{"y (mm)", ""}, {"x (mm)",
      StringJoin@RandomChoice[CharacterRange["a", "z"], 20]}},
   BaseStyle -> {FontWeight -> "Bold", FontSize -> 40,
     FontFamily -> "Calibri"}, ImageSize -> 2000], "PNG"], "PNG"]

Export["export23.png", Cell@BoxData@ToBoxes[plot]]

Export["export23.png", RawBoxes@ToBoxes[plot]]

Export["export23.png", ExpressionCell[plot]]